xfs: remove xfs_page_mkwrite_iomap_ops

Shared the regular buffered write iomap_ops with the page fault path
and just check for the IOMAP_FAULT flag to skip delalloc punching.

This keeps the delalloc punching checks in one place, and will make it
easier to convert iomap to an iter model where the begin and end
handlers are merged into a single callback.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>