btrfs-progs: subvol show: add support to search subvolume by rootid or uuid

btrfs-progs: subvol show: add support to search subvolume by rootid or uuid

Unless the top level is mounted there is no way to know the
details of all the subvolume.  For example:

mount -o subvol=sv1/newsv1 /dev/sdb /btrfs

btrfs su list /btrfs
ID 257 gen 12 top level 5 path sv1
ID 258 gen 9 top level 257 path sv1/snap
ID 259 gen 11 top level 257 path sv1/newsv1

You can't subvol show for sv1 and sv1/snap as its paths aren't
accessible to the user unless the its top level is mounted.

This patch adds two new options to the existing btrfs subvol show
cli. They are --rootid/-r or --uuid/-u, with this now the user will
be able to look for a subvolume using the rootid OR the uuid.

./btrfs su show -r 257 /btrfs
sv1
	Name: 			sv1
	UUID: 			30129358-c69d-3e4a-a662-29509cc69c95
	Parent UUID: 		-
	Received UUID: 		-
	Creation time: 		2017-07-11 20:32:57 +0800
	Subvolume ID: 		257
	Generation: 		12
	Gen at creation: 	7
	Parent ID: 		5
	Top level ID: 		5
	Flags: 			-
	Snapshot(s):
				sv1/snap

Signed-off-by: Anand Jain <anand.jain@oracle.com>
[ minor adjustments in the help text ]
Signed-off-by: David Sterba <dsterba@suse.com>