xfs: don't look at buffer heads in xfs_add_to_ioend

Calculate all information for the bio based on the passed in information
without requiring a buffer_head structure.

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>