xfs: refactor the tail of xfs_writepage_map

Rejuggle how we deal with the different error vs non-error and have
ioends vs not have ioend cases to keep the fast path streamlined, and
the duplicate code at a minimum.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>