xfs: add initial DAX support

xfs: add initial DAX support

Orabug: 22913653

Add initial DAX support to XFS. To do this we need a new mount
option to turn DAX on filesystem, and we need to propagate this into
the inode flags whenever an inode is instantiated so that the
per-inode checks throughout the code Do The Right Thing.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>
(cherry picked from commit cbe4dab119f211ff6642d617f541087894e99e4f)
Signed-off-by: Dan Duval <dan.duval@oracle.com>