iomap: take responsibility for the filesystem iomap code

XFS is the primary user of the filesystem iomap code, so we (the XFS
community, specifically) will explicitly take responsibility for its
maintenance.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Acked-by: Christoph Hellwig <hch@lst.de>