xfs: add ->pfn_mkwrite support for DAX

xfs: add ->pfn_mkwrite support for DAX

Orabug: 22913653

->pfn_mkwrite support is needed so that when a page with allocated
backing store takes a write fault we can check that the fault has
not raced with a truncate and is pointing to a region beyond the
current end of file.

This also allows us to update the timestamp on the inode, too, which
fixes a generic/080 failure.

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 3af49285854df66260a263198cc15abb07b95287)
Signed-off-by: Dan Duval <dan.duval@oracle.com>