IB/rdmavt: Don't vfree a kzalloc'ed memory region

The userspace memory region 'mr' is allocated with kzalloc in
__rvt_alloc_mr  however it is incorrectly being freed with vfree in
__rvt_free_mr. Fix this by using kfree to free it.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Acked-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>