cm.lock must be held while accessing remote_sidr_table. This fixes the
below NULL pointer dereference.

  BUG: kernel NULL pointer dereference, address: 0000000000000000
  #PF: supervisor write access in kernel mode
  #PF: error_code(0x0002) - not-present page
  PGD 0 P4D 0
  Oops: 0002 [#1] SMP PTI
  CPU: 2 PID: 7288 Comm: udaddy Not tainted 5.7.0_for_upstream_perf_2020_06_09_15_14_20_38 #1
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.12.1-0-ga5cab58e9a3f-prebuilt.qemu.org 04/01/2014
  RIP: 0010:rb_erase+0x10d/0x360
  Code: 00 00 00 48 89 c1 48 89 d0 48 8b 50 08 48 39 ca 74 48 f6 02 01 75 af 48 8b 7a 10 48 89 c1 48 83 c9 01 48 89 78 08 48 89 42 10 <48> 89 0f 48 8b 08 48 89 0a 48 83 e1 fc 48 89 10 0f 84 b1 00 00 00
  RSP: 0018:ffffc90000f77c30 EFLAGS: 00010086
  RAX: ffff8883df27d458 RBX: ffff8883df27da58 RCX: ffff8883df27d459
  RDX: ffff8883d183fa58 RSI: ffffffffa01e8d00 RDI: 0000000000000000
  RBP: ffff8883d62ac800 R08: 0000000000000000 R09: 00000000000000ce
  R10: 000000000000000a R11: 0000000000000000 R12: ffff8883df27da00
  R13: ffffc90000f77c98 R14: 0000000000000130 R15: 0000000000000000
  FS:  00007f009f877740(0000) GS:ffff8883f1a00000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 0000000000000000 CR3: 00000003d467e003 CR4: 0000000000160ee0
  Call Trace:
   cm_send_sidr_rep_locked+0x15a/0x1a0 [ib_cm]
   ib_send_cm_sidr_rep+0x2b/0x50 [ib_cm]
   cma_send_sidr_rep+0x8b/0xe0 [rdma_cm]
   __rdma_accept+0x21d/0x2b0 [rdma_cm]
   ? ucma_get_ctx+0x2b/0xe0 [rdma_ucm]
   ? _copy_from_user+0x30/0x60
   ucma_accept+0x13e/0x1e0 [rdma_ucm]
   ucma_write+0xb4/0x130 [rdma_ucm]
   vfs_write+0xad/0x1a0
   ksys_write+0x9d/0xb0
   do_syscall_64+0x48/0x130
   entry_SYSCALL_64_after_hwframe+0x44/0xa9
  RIP: 0033:0x7f009ef60924
  Code: 00 f7 d8 64 89 02 48 c7 c0 ff ff ff ff eb b7 0f 1f 80 00 00 00 00 8b 05 2a ef 2c 00 48 63 ff 85 c0 75 13 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 54 f3 c3 66 90 55 53 48 89 d5 48 89 f3 48 83
  RSP: 002b:00007fff843edf38 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
  RAX: ffffffffffffffda RBX: 000055743042e1d0 RCX: 00007f009ef60924
  RDX: 0000000000000130 RSI: 00007fff843edf40 RDI: 0000000000000003
  RBP: 00007fff843ee0e0 R08: 0000000000000000 R09: 0000557430433090
  R10: 0000000000000001 R11: 0000000000000246 R12: 0000000000000000
  R13: 00007fff843edf40 R14: 000000000000038c R15: 00000000ffffff00
  CR2: 0000000000000000

Fixes: 6a8824a7 ("RDMA/cm: Allow ib_send_cm_sidr_rep() to be done under lock")
Link: https://lore.kernel.org/r/20200716105519.1424266-1-leon@kernel.orgSigned-off-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>