netfs: Mark netfs_unbuffered_write_iter_locked() static

Mark netfs_unbuffered_write_iter_locked() static as it's only called from
the file in which it is defined.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
cc: linux-fsdevel@vger.kernel.org
cc: linux-mm@kvack.org