netfs: Implement RMW read-side

Implement the read part of RMW.  This will be necessary to handle such
things as version conflicts when updating a partial write to a
content-encrypted file when doing unbuffered writes.

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