In nouveau_dmem_pages_alloc(), the drm->dmem->mutex is unlocked before
calling nouveau_dmem_chunk_alloc() as shown when CONFIG_PROVE_LOCKING
is enabled:

[ 1294.871933] =====================================
[ 1294.876656] WARNING: bad unlock balance detected!
[ 1294.881375] 5.2.0-rc3+ #5 Not tainted
[ 1294.885048] -------------------------------------
[ 1294.889773] test-malloc-vra/6299 is trying to release lock (&drm->dmem->mutex) at:
[ 1294.897482] [<ffffffffa01a220f>] nouveau_dmem_migrate_alloc_and_copy+0x79f/0xbf0 [nouveau]
[ 1294.905782] but there are no more locks to release!
[ 1294.910690]
[ 1294.910690] other info that might help us debug this:
[ 1294.917249] 1 lock held by test-malloc-vra/6299:
[ 1294.921881]  #0: 0000000016e10454 (&mm->mmap_sem#2){++++}, at: nouveau_svmm_bind+0x142/0x210 [nouveau]
[ 1294.931313]
[ 1294.931313] stack backtrace:
[ 1294.935702] CPU: 4 PID: 6299 Comm: test-malloc-vra Not tainted 5.2.0-rc3+ #5
[ 1294.942786] Hardware name: ASUS X299-A/PRIME X299-A, BIOS 1401 05/21/2018
[ 1294.949590] Call Trace:
[ 1294.952059]  dump_stack+0x7c/0xc0
[ 1294.955469]  ? nouveau_dmem_migrate_alloc_and_copy+0x79f/0xbf0 [nouveau]
[ 1294.962213]  print_unlock_imbalance_bug.cold.52+0xca/0xcf
[ 1294.967641]  lock_release+0x306/0x380
[ 1294.971383]  ? nouveau_dmem_migrate_alloc_and_copy+0x79f/0xbf0 [nouveau]
[ 1294.978089]  ? lock_downgrade+0x2d0/0x2d0
[ 1294.982121]  ? find_held_lock+0xac/0xd0
[ 1294.985979]  __mutex_unlock_slowpath+0x8f/0x3f0
[ 1294.990540]  ? wait_for_completion+0x230/0x230
[ 1294.995002]  ? rwlock_bug.part.2+0x60/0x60
[ 1294.999197]  nouveau_dmem_migrate_alloc_and_copy+0x79f/0xbf0 [nouveau]
[ 1295.005751]  ? page_mapping+0x98/0x110
[ 1295.009511]  migrate_vma+0xa74/0x1090
[ 1295.013186]  ? move_to_new_page+0x480/0x480
[ 1295.017400]  ? __kmalloc+0x153/0x300
[ 1295.021052]  ? nouveau_dmem_migrate_vma+0xd8/0x1e0 [nouveau]
[ 1295.026796]  nouveau_dmem_migrate_vma+0x157/0x1e0 [nouveau]
[ 1295.032466]  ? nouveau_dmem_init+0x490/0x490 [nouveau]
[ 1295.037612]  ? vmacache_find+0xc2/0x110
[ 1295.041537]  nouveau_svmm_bind+0x1b4/0x210 [nouveau]
[ 1295.046583]  ? nouveau_svm_fault+0x13e0/0x13e0 [nouveau]
[ 1295.051912]  drm_ioctl_kernel+0x14d/0x1a0
[ 1295.055930]  ? drm_setversion+0x330/0x330
[ 1295.059971]  drm_ioctl+0x308/0x530
[ 1295.063384]  ? drm_version+0x150/0x150
[ 1295.067153]  ? find_held_lock+0xac/0xd0
[ 1295.070996]  ? __pm_runtime_resume+0x3f/0xa0
[ 1295.075285]  ? mark_held_locks+0x29/0xa0
[ 1295.079230]  ? _raw_spin_unlock_irqrestore+0x3c/0x50
[ 1295.084232]  ? lockdep_hardirqs_on+0x17d/0x250
[ 1295.088768]  nouveau_drm_ioctl+0x9a/0x100 [nouveau]
[ 1295.093661]  do_vfs_ioctl+0x137/0x9a0
[ 1295.097341]  ? ioctl_preallocate+0x140/0x140
[ 1295.101623]  ? match_held_lock+0x1b/0x230
[ 1295.105646]  ? match_held_lock+0x1b/0x230
[ 1295.109660]  ? find_held_lock+0xac/0xd0
[ 1295.113512]  ? __do_page_fault+0x324/0x630
[ 1295.117617]  ? lock_downgrade+0x2d0/0x2d0
[ 1295.121648]  ? mark_held_locks+0x79/0xa0
[ 1295.125583]  ? handle_mm_fault+0x352/0x430
[ 1295.129687]  ksys_ioctl+0x60/0x90
[ 1295.133020]  ? mark_held_locks+0x29/0xa0
[ 1295.136964]  __x64_sys_ioctl+0x3d/0x50
[ 1295.140726]  do_syscall_64+0x68/0x250
[ 1295.144400]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
[ 1295.149465] RIP: 0033:0x7f1a3495809b
[ 1295.153053] Code: 0f 1e fa 48 8b 05 ed bd 0c 00 64 c7 00 26 00 00 00 48 c7 c0 ff ff ff ff c3 66 0f 1f 44 00 00 f3 0f 1e fa b8 10 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d bd bd 0c 00 f7 d8 64 89 01 48
[ 1295.171850] RSP: 002b:00007ffef7ed1358 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
[ 1295.179451] RAX: ffffffffffffffda RBX: 00007ffef7ed1628 RCX: 00007f1a3495809b
[ 1295.186601] RDX: 00007ffef7ed13b0 RSI: 0000000040406449 RDI: 0000000000000004
[ 1295.193759] RBP: 00007ffef7ed13b0 R08: 0000000000000000 R09: 000000000157e770
[ 1295.200917] R10: 000000000151c010 R11: 0000000000000246 R12: 0000000040406449
[ 1295.208083] R13: 0000000000000004 R14: 0000000000000000 R15: 0000000000000000

Reacquire the lock before continuing to the next page.
Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>