nfs_get_lock_context can return NULL on an allocation failure.
Regression introduced by commit f11ac8db.
Reported-by: NSteve Dickson <steved@redhat.com>
Signed-off-by: NJeff Layton <jlayton@redhat.com>
Cc: stable@kernel.org
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>