netfs, cifs: Move CIFS_INO_MODIFIED_ATTR to netfs_inode

Move CIFS_INO_MODIFIED_ATTR to netfs_inode as NETFS_ICTX_MODIFIED_ATTR and
then make netfs_perform_write() set it.  This means that cifs doesn't need
to implement the ->post_modify() hook.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: Steve French <sfrench@samba.org>
cc: Paulo Alcantara <pc@manguebit.com>
cc: linux-cifs@vger.kernel.org
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Link: https://lore.kernel.org/r/20240814203850.2240469-7-dhowells@redhat.com/

 # v2
Signed-off-by: Christian Brauner <brauner@kernel.org>