vfs: Add iomap_seek_hole and iomap_seek_data helpers

Filesystems can use this for implementing lseek SEEK_HOLE / SEEK_DATA
support via iomap.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
[hch: split functions, coding style cleanups]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>