iomap: replace iomap_folio_ops with iomap_write_ops

iomap: replace iomap_folio_ops with iomap_write_ops

The iomap_folio_ops are only used for buffered writes, including the zero
and unshare variants.  Rename them to iomap_write_ops to better describe
the usage, and pass them through the call chain like the other operation
specific methods instead of through the iomap.

xfs_iomap_valid grows a IOMAP_HOLE check to keep the existing behavior
that never attached the folio_ops to a iomap representing a hole.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Acked-by: Damien Le Moal <dlemoal@kernel.org>