syzkaller hits a WARN_ON when trying to have a uptr close to UINTPTR_MAX:

  WARNING: CPU: 1 PID: 393 at drivers/iommu/iommufd/selftest.c:403 iommufd_test+0xb19/0x16f0
  Modules linked in:
  CPU: 1 PID: 393 Comm: repro Not tainted 6.2.0-c9c3395d #1
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014
  RIP: 0010:iommufd_test+0xb19/0x16f0
  Code: 94 c4 31 ff 44 89 e6 e8 a5 54 17 ff 45 84 e4 0f 85 bb 0b 00 00 41 be fb ff ff ff e8 31 53 17 ff e9 a0 f7 ff ff e8 27 53 17 ff <0f> 0b 41 be 8
  RSP: 0018:ffffc90000eabdc0 EFLAGS: 00010246
  RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffffff8214c487
  RDX: 0000000000000000 RSI: ffff88800f5c8000 RDI: 0000000000000002
  RBP: ffffc90000eabe48 R08: 0000000000000000 R09: 0000000000000001
  R10: 0000000000000001 R11: 0000000000000000 R12: 00000000cd2b0000
  R13: 00000000cd2af000 R14: 0000000000000000 R15: ffffc90000eabe68
  FS:  00007f94d76d5740(0000) GS:ffff88807dd00000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 0000000020000043 CR3: 0000000006880006 CR4: 0000000000770ee0
  PKRU: 55555554
  Call Trace:
   <TASK>
   ? write_comp_data+0x2f/0x90
   iommufd_fops_ioctl+0x1ef/0x310
   __x64_sys_ioctl+0x10e/0x160
   ? __pfx_iommufd_fops_ioctl+0x10/0x10
   do_syscall_64+0x3b/0x90
   entry_SYSCALL_64_after_hwframe+0x72/0xdc

Check that the user memory range doesn't overflow.

Fixes: f4b20bb3 ("iommufd: Add kernel support for testing iommufd")
Link: https://lore.kernel.org/r/0-v1-95390ed1df8d+8f-iommufd_mock_overflow_jgg@nvidia.comReviewed-by: Kevin Tian <kevin.tian@intel.com>
Reported-by: Pengfei Xu <pengfei.xu@intel.com>
Link: https://lore.kernel.org/r/Y/hOiilV1wJvu/Hv@xpf.sh.intel.comSigned-off-by: Jason Gunthorpe <jgg@nvidia.com>