Set mr->umem to NULL in mlx4_ib_alloc_fast_reg_mr(). Otherwise
ib_dereg_mr() may invoke ib_umem_release() on a random pointer value
and get an oops.
Signed-off-by: NVladimir Sokolovsky <vlad@mellanox.co.il>
Signed-off-by: NRoland Dreier <rolandd@cisco.com>