btrfs-progs: show extent state for a subvolume

The qgroup verification code can trivially be extended to provide
extended information on the extents which a subvolume root
references. Along with qgroup-verify, I have found this tool to be
invaluable when tracking down extent references.

The patch adds a switch to the check subcommand '--subvol-extents'
which takes as args a single subvolume id. When run with the switch,
we'll print out each extent that the subvolume references. The extent
printout gives standard extent info you would expect along with
information on which other roots reference it.

Sample output follows - this is a few lines from a run on a subvolume
I've been testing qgroup changes on:

Print extent state for subvolume 281 on /dev/vdb2
UUID: 8203ca66-9858-4e3f-b447-5bbaacf79c02
Offset		Len	Root Refs	Roots
12582912	20480	12	257 279 280 281 282 283 284 285 286 287 288 289
12603392	8192	12	257 279 280 281 282 283 284 285 286 287 288 289
12611584	12288	12	257 279 280 281 282 283 284 285 286 287 288 289
 <snip a bunch of extents to show some variety>
124583936	16384	4	281 282 283 280
125075456	16384	4	280 281 282 283
126255104	16384	11	257 280 281 282 283 284 285 286 287 288 289
4763508736	4096	3	279 280 281

In case it wasn't clear, this applies on top of my qgroup verify patch:
"btrfs-progs: add quota group verify code"

A branch with all this can be found on github:

https://github.com/markfasheh/btrfs-progs-patches/tree/qgroup-verify

Please apply,

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