nfs: handle lock context allocation failures in nfs_create_request

nfs_get_lock_context can return NULL on an allocation failure.
Regression introduced by commit f11ac8db.

Reported-by: Steve Dickson <steved@redhat.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Cc: stable@kernel.org
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>