netfs: Add a hook to allow tell the netfs to update its i_size

Add a hook for netfslib's write helpers to call to tell the network
filesystem that it should update its i_size.

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