btrfs: don't update mtime/ctime on deduped inodes

One issue users have reported is that dedupe changes mtime on files,
resulting in tools like rsync thinking that their contents have changed when
in fact the data is exactly the same. We also skip the ctime update as no
user-visible metadata changes here and we want dedupe to be transparent to
the user.

Clone still wants time changes, so we special case this in the code.

This was tested with the btrfs-extent-same tool.

Signed-off-by: Mark Fasheh <mfasheh@suse.de>
Signed-off-by: Chris Mason <clm@fb.com>