RDMA/core: Prevent divide-by-zero error triggered by the user

RDMA/core: Prevent divide-by-zero error triggered by the user

The user_entry_size is supplied by the user and later used as a
denominator to calculate number of entries. The zero supplied by the user
will trigger the following divide-by-zero error:

 divide error: 0000 [#1] SMP KASAN PTI
 CPU: 4 PID: 497 Comm: c_repro Not tainted 5.13.0-rc1+ #281
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.13.0-0-gf21b5a4aeb02-prebuilt.qemu.org 04/01/2014
 RIP: 0010:ib_uverbs_handler_UVERBS_METHOD_QUERY_GID_TABLE+0x1b1/0x510
 Code: 87 59 03 00 00 e8 9f ab 1e ff 48 8d bd a8 00 00 00 e8 d3 70 41 ff 44 0f b7 b5 a8 00 00 00 e8 86 ab 1e ff 31 d2 4c 89 f0 31 ff <49> f7 f5 48 89 d6 48 89 54 24 10 48 89 04 24 e8 1b ad 1e ff 48 8b
 RSP: 0018:ffff88810416f828 EFLAGS: 00010246
 RAX: 0000000000000008 RBX: 1ffff1102082df09 RCX: ffffffff82183f3d
 RDX: 0000000000000000 RSI: ffff888105f2da00 RDI: 0000000000000000
 RBP: ffff88810416fa98 R08: 0000000000000001 R09: ffffed102082df5f
 R10: ffff88810416faf7 R11: ffffed102082df5e R12: 0000000000000000
 R13: 0000000000000000 R14: 0000000000000008 R15: ffff88810416faf0
 FS:  00007f5715efa740(0000) GS:ffff88811a700000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 0000000020000840 CR3: 000000010c2e0001 CR4: 0000000000370ea0
 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
 Call Trace:
  ? ib_uverbs_handler_UVERBS_METHOD_INFO_HANDLES+0x4b0/0x4b0
  ib_uverbs_cmd_verbs+0x1546/0x1940
  ib_uverbs_ioctl+0x186/0x240
  __x64_sys_ioctl+0x38a/0x1220
  do_syscall_64+0x3f/0x80
  entry_SYSCALL_64_after_hwframe+0x44/0xae

Fixes: 9f85cbe50aa0 ("RDMA/uverbs: Expose the new GID query API to user space")
Link: https://lore.kernel.org/r/b971cc70a8b240a8b5eda33c99fa0558a0071be2.1620657876.git.leonro@nvidia.com
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>