RDMA/uverbs: Store PR pointer before it is overwritten

RDMA/uverbs: Store PR pointer before it is overwritten

The IB_MR_REREG_PD command rewrites mr->pd after successful
rereg_user_mr(), such change causes to lost usecnt information and
produces the following warning:

 WARNING: CPU: 1 PID: 1771 at drivers/infiniband/core/verbs.c:336 ib_dealloc_pd+0x4e/0x60 [ib_core]
 CPU: 1 PID: 1771 Comm: rereg_mr Tainted: G        W  OE 5.0.0-rc7-for-upstream-perf-2019-02-20_14-03-40-34 #1
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1ubuntu1 04/01/2014
 RIP: 0010:ib_dealloc_pd+0x4e/0x60 [ib_core]
 RSP: 0018:ffffc90003923dc0 EFLAGS: 00010286
 RAX: 00000000ffffffff RBX: ffff88821f7f0400 RCX: ffff888236a40c00
 RDX: ffff88821f7f0400 RSI: 0000000000000001 RDI: 0000000000000000
 RBP: 0000000000000001 R08: ffff88835f665d80 R09: ffff8882209c90d8
 R10: ffff88835ec003e0 R11: 0000000000000000 R12: ffff888221680ba0
 R13: ffff888221680b00 R14: 00000000ffffffea R15: ffff88821f53c318
 FS:  00007f70db11e740(0000) GS:ffff88835f640000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 0000000001dfd030 CR3: 000000029d9d8000 CR4: 00000000000006e0
 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
 Call Trace:
  uverbs_free_pd+0x2d/0x30 [ib_uverbs]
  destroy_hw_idr_uobject+0x16/0x40 [ib_uverbs]
  uverbs_destroy_uobject+0x28/0x170 [ib_uverbs]
  __uverbs_cleanup_ufile+0x6b/0x90 [ib_uverbs]
  uverbs_destroy_ufile_hw+0x8b/0x110 [ib_uverbs]
  ib_uverbs_close+0x1f/0x80 [ib_uverbs]
  __fput+0xb1/0x220
  task_work_run+0x7f/0xa0
  exit_to_usermode_loop+0x6b/0xb2
  do_syscall_64+0xc5/0x100
  entry_SYSCALL_64_after_hwframe+0x44/0xa9
 RIP: 0033:0x7f70dad00664

Fixes: e278173fd19e ("RDMA/core: Cosmetic change - move member initialization to correct block")
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Majd Dibbiny <majd@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>