RDMA/uverbs: Fix kernel panic while using XRC_TGT QP type

Attempt to modify XRC_TGT QP type from the user space (ibv_xsrq_pingpong
invocation) will trigger the following kernel panic. It is caused by the
fact that such QPs missed uobject initialization.

[   17.408845] BUG: unable to handle kernel NULL pointer dereference at 0000000000000048
[   17.412645] IP: rdma_lookup_put_uobject+0x9/0x50
[   17.416567] PGD 0 P4D 0
[   17.419262] Oops: 0000 [#1] SMP PTI
[   17.422915] CPU: 0 PID: 455 Comm: ibv_xsrq_pingpo Not tainted 4.16.0-rc1+ #86
[   17.424765] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.7.5-0-ge51488c-20140602_164612-nilsson.home.kraxel.org 04/01/2014
[   17.427399] RIP: 0010:rdma_lookup_put_uobject+0x9/0x50
[   17.428445] RSP: 0018:ffffb8c7401e7c90 EFLAGS: 00010246
[   17.429543] RAX: 0000000000000000 RBX: ffffb8c7401e7cf8 RCX: 0000000000000000
[   17.432426] RDX: 0000000000000001 RSI: 0000000000000000 RDI: 0000000000000000
[   17.437448] RBP: 0000000000000000 R08: 00000000000218f0 R09: ffffffff8ebc4cac
[   17.440223] R10: fffff6038052cd80 R11: ffff967694b36400 R12: ffff96769391f800
[   17.442184] R13: ffffb8c7401e7cd8 R14: 0000000000000000 R15: ffff967699f60000
[   17.443971] FS:  00007fc29207d700(0000) GS:ffff96769fc00000(0000) knlGS:0000000000000000
[   17.446623] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   17.448059] CR2: 0000000000000048 CR3: 000000001397a000 CR4: 00000000000006b0
[   17.449677] Call Trace:
[   17.450247]  modify_qp.isra.20+0x219/0x2f0
[   17.451151]  ib_uverbs_modify_qp+0x90/0xe0
[   17.452126]  ib_uverbs_write+0x1d2/0x3c0
[   17.453897]  ? __handle_mm_fault+0x93c/0xe40
[   17.454938]  __vfs_write+0x36/0x180
[   17.455875]  vfs_write+0xad/0x1e0
[   17.456766]  SyS_write+0x52/0xc0
[   17.457632]  do_syscall_64+0x75/0x180
[   17.458631]  entry_SYSCALL_64_after_hwframe+0x21/0x86
[   17.460004] RIP: 0033:0x7fc29198f5a0
[   17.460982] RSP: 002b:00007ffccc71f018 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
[   17.463043] RAX: ffffffffffffffda RBX: 0000000000000078 RCX: 00007fc29198f5a0
[   17.464581] RDX: 0000000000000078 RSI: 00007ffccc71f050 RDI: 0000000000000003
[   17.466148] RBP: 0000000000000000 R08: 0000000000000078 R09: 00007ffccc71f050
[   17.467750] R10: 000055b6cf87c248 R11: 0000000000000246 R12: 00007ffccc71f300
[   17.469541] R13: 000055b6cf8733a0 R14: 0000000000000000 R15: 0000000000000000
[   17.471151] Code: 00 00 0f 1f 44 00 00 48 8b 47 48 48 8b 00 48 8b 40 10 e9 0b 8b 68 00 90 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 55 53 89 f5 <48> 8b 47 48 48 89 fb 40 0f b6 f6 48 8b 00 48 8b 40 20 e8 e0 8a
[   17.475185] RIP: rdma_lookup_put_uobject+0x9/0x50 RSP: ffffb8c7401e7c90
[   17.476841] CR2: 0000000000000048
[   17.477764] ---[ end trace 1dbcc5354071a712 ]---
[   17.478880] Kernel panic - not syncing: Fatal exception
[   17.480277] Kernel Offset: 0xd000000 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff)

Fixes: 2f08ee36 ("RDMA/restrack: don't use uaccess_kernel()")
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>