xfs: factor mapping out of xfs_do_writepage

xfs: factor mapping out of xfs_do_writepage

Separate out the bufferhead based mapping from the writepage code so
that we have a clear separation of the page operations and the
bufferhead state.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>
(cherry picked from commit bfce7d2e2d5ee05e9d465888905c66a70a9c243c)

Orabug: 28193043

Signed-off-by: Shan Hai <shan.hai@oracle.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>