RDMA/rxe: Fix slab-out-bounds access which lead to kernel crash later

BUG: KASAN: slab-out-of-bounds in rxe_mem_init_user+0x6c1/0x740 [rdma_rxe]
Read of size 8 at addr ffff88805c01a608 by task ib_send_bw/573

CPU: 24 PID: 573 Comm: ib_send_bw Not tainted 5.0.0-rc5+ #189
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.11.0-0-g63451fca13-prebuilt.qemu-project.org 04/01/2014
Call Trace:
 rxe_mem_init_user+0x6c1/0x740 [rdma_rxe]
 rxe_reg_user_mr+0x9b/0x110 [rdma_rxe]
 ib_uverbs_reg_mr+0x428/0x9c0 [ib_uverbs]
 ib_uverbs_handler_UVERBS_METHOD_INVOKE_WRITE+0x2b0/0x410 [ib_uverbs]
 ib_uverbs_run_method+0x79c/0x1da0 [ib_uverbs]
 rxe_mem_init_user+0x6c1/0x740 [rdma_rxe]
 rxe_reg_user_mr+0x9b/0x110 [rdma_rxe]
 ib_uverbs_reg_mr+0x428/0x9c0 [ib_uverbs]
 ib_uverbs_handler_UVERBS_METHOD_INVOKE_WRITE+0x2b0/0x410 [ib_uverbs]
 ib_uverbs_run_method+0x79c/0x1da0 [ib_uverbs]
 ib_uverbs_cmd_verbs+0x5f2/0xf20 [ib_uverbs]
 ib_uverbs_ioctl+0x202/0x310 [ib_uverbs]
 do_vfs_ioctl+0x193/0x1440
 ksys_ioctl+0x3a/0x70
 __x64_sys_ioctl+0x6f/0xb0
 do_syscall_64+0x13f/0x570
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

Allocated by task 573:
 __kasan_kmalloc.constprop.5+0xc1/0xd0
 __kmalloc+0x161/0x310
 rxe_mem_alloc+0x52/0x470 [rdma_rxe]
 rxe_mem_init_user+0x113/0x740 [rdma_rxe]
 rxe_reg_user_mr+0x9b/0x110 [rdma_rxe]
 ib_uverbs_reg_mr+0x428/0x9c0 [ib_uverbs]
 ib_uverbs_handler_UVERBS_METHOD_INVOKE_WRITE+0x2b0/0x410 [ib_uverbs]
 ib_uverbs_run_method+0x79c/0x1da0 [ib_uverbs]
 ib_uverbs_cmd_verbs+0x5f2/0xf20 [ib_uverbs]
 ib_uverbs_ioctl+0x202/0x310 [ib_uverbs]
 do_vfs_ioctl+0x193/0x1440
 ksys_ioctl+0x3a/0x70
 __x64_sys_ioctl+0x6f/0xb0
 do_syscall_64+0x13f/0x570
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

Freed by task 0:
 __kasan_slab_free+0x12e/0x180
 kfree+0x10a/0x2c0
 rcu_process_callbacks+0xa77/0x1260
 __do_softirq+0x2ad/0xacb

Test scenario:
 ib_send_bw -x 1 -d rxe0 -a &
 ib_send_bw -x 1 -d rxe0 -a localhost

Fixes: 8700e3e7 ("Soft RoCE driver")
Reported-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Zhu Yanjun <yanjun.zhu@oracle.com>
Tested-by: Zhu Yanjun <yanjun.zhu@oracle.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>