dax: improve comment about truncate race

dax: improve comment about truncate race

Orabug: 22913653

Jan Kara pointed out I should be more explicit here about the perils of
racing against truncate.  The comment is mostly the same as for the PTE
case.

Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit 84c4e5e675408b6fb7d74eec7da9a4a5698b50af)
Signed-off-by: Dan Duval <dan.duval@oracle.com>