make "btrfs filesystem label" command actually work

make "btrfs filesystem label" command actually work

This simple patch makes "btrfs filesystem label" command actually work.

On tmp branch, commit d1dc6a9, "btrfs filesystem label" functionality
was introduced. However the commit lacks one component that lets
"btrfs" accept "filesystem label" command.
Test case:

/dev/loop0

WARNING! - Btrfs Btrfs v0.19 IS EXPERIMENTAL
WARNING! - see http://btrfs.wiki.kernel.org before using

fs created label old on /dev/loop0
	nodesize 4096 leafsize 4096 sectorsize 4096 size 1.00GB
Btrfs Btrfs v0.19

old

new

FATAL: the filesystem has to be unmounted

new

Not sure if you need if you need a signoff for something as trivial as
this, but here it is just in case.

Signed-off-by: Fajar A. Nugraha <list@fajar.net>
Signed-off-by: Hugo Mills <hugo@carfax.org.uk>