RFC btrfs-progs: snapshot diff function

Hello,

I've done a prototype implementation of snapshot diff utility many months ago. It was originally meant to analyze the differences between two snapshots which are inherited from the same subvolume/snapshot.

Moreover, the upstream LXC userland tools has been released with a dedicated template to create new containers combine with btrfs subvolume/snapshot create function, so this path set might be useful if someone is suffering from a broken container guest(it was cloned from a health one in previous but it does not work with some configurations now). In this case, this feature could works as an assistant to help investigating the root cause by listing those changed files from the snapshot that the container resides.

This patch set works to three kinds of change for now.
- new_file: new created files at the destination snapshot.

- removed_file: those files are still resides on source subvolume/snapshot but they have been removed from the destination.

- updated_file: files are resides on both subvolumes/snapshots, but they might be changed.

Currently, the user could do diff business on any two subvolumes/snapshots, if the destination snapshot is not inherited from the same subvolume/snapshot upon the source one, he must be surprised by the results, so it's better to improve it with pre-check for that if possible.

Another issue is,
- if we created some new files or updated some existing files under the source snapshot, they will be marked as REMOVED/UPDATED out of the source from the destination snapshot's point of view, so the results might looks a bit strange.

A quick demo:

root@kdev:/btrfs# btrfs subvolume diff-snapshot one two            
[REMOVED REGFILE] one/regfile_in_one objectid 264 transid 50
[REMOVED DIR] one/dir_02_at_one objectid 262 transid 36
[REMOVED REGFILE] one/dir_02_at_one/file_at_dir02_one objectid 263 transid 37
[REMOVED DIR] one/dir_at_one objectid 258 transid 29
[REMOVED REGFILE] one/dir_at_one/file_02_at_one_dir objectid 260 transid 32
[REMOVED REGFILE] one/dir_at_one/file_03_at_one_dir objectid 261 transid 35
[REMOVED REGFILE] one/dir_at_one/file_at_one_dir objectid 259 transid 30
[REMOVED REGFILE] one/file_at_one objectid 257 transid 26
[NEW REGFILE] two/regfile_in_two objectid 265 transid 50
[NEW DIR] two/dir_at_two objectid 262 transid 40
[NEW REGFILE] two/dir_at_two/file01_at_dir_of_two objectid 263 transid 41
[NEW SYMLINK] two/dir_at_two/passwd objectid 264 transid 42
[NEW REGFILE] two/file_02 objectid 258 transid 23
[NEW REGFILE] two/file_03 objectid 270 transid 68
[NEW REGFILE] two/file_04 objectid 275 transid 68

Any comments are appreciated!

Thanks,
-Jeff

 Makefile         |    2 +-
 btrfs-list.c     |    3 +-
 cmds-subvolume.c |   90 +++++
 diff-snapshot.c  | 1026 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 diff-snapshot.h  |   47 +++
 5 files changed, 1165 insertions(+), 3 deletions(-)