IB/rxe: Fix a MR reference leak in check_rkey()

Avoid that calling check_rkey() for mem->state == RXE_MEM_STATE_FREE
triggers an MR reference leak.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: Andrew Boyer <andrew.boyer@dell.com>
Cc: Moni Shoua <monis@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>