RDMA/ucma: Check that device exists prior to accessing it

commit c8d3bcbfc5eab3f01cf373d039af725f3b488813 upstream.

Ensure that device exists prior to accessing its properties.

Reported-by:  <syzbot+71655d44855ac3e76366@syzkaller.appspotmail.com>
Fixes: 75216638 ("RDMA/cma: Export rdma cm interface to userspace")
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>