iomap: Permit pages without an iop to enter writeback

Create an iop in the writeback path if one doesn't exist.  This allows us
to avoid creating the iop in some cases.  We'll initially do that for pages
with inline data, but it can be extended to pages which are entirely within
an extent.  It also allows for an iop to be removed from pages in the
future (eg page split).

Co-developed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>