Btrfs-progs: add function to map subvol ID to path

Btrfs-progs: add function to map subvol ID to path

Several tools like btrfs-send and btrfs-receive need to map a
subvolume ID to a filesystem path. The so far existing methods
in btrfs-list.c cause a horrible effort when performing this
operation (and the effort is dependent on the number of
existing subvolumes with quadratic effort). This commit adds a
function that is able to map a subvolume ID to a filesystem path
with an effort that is independent of the number of existing
subvolumes.

In addition to this function, a command line frontend is added as well:
btrfs inspect-internal subvolid-resolve <subvolid> <path>

Signed-off-by: Stefan Behrens <sbehrens@giantdisaster.de>