btrfs: add no_mtime flag to btrfs-extent-same

btrfs: add no_mtime flag to btrfs-extent-same

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. Clone still wants an mtime change, so
we special case this in the code.

With this patch an application can pass the BTRFS_SAME_NO_MTIME flag to a
dedupe request and the kernel will honor it by only changing ctime.

I have an updated version of the btrfs-extent-same test program with a
switch to provide this flag at the 'no_time' branch of:

https://github.com/markfasheh/duperemove/

Signed-off-by: Mark Fasheh <mfasheh@suse.de>