fs: Export generic_atomic_write_valid()

The XFS code will need this.

Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: John Garry <john.g.garry@oracle.com>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>