libdisk: recognize btrfs disk format

libdisk: recognize btrfs disk format

Now that the btrfs disk format has stabilized, libdisk can
recognize it and mkfs.xfs will not overwrite it w/o "-f" :

# mkfs.btrfs -l 16384 -L mylabel -n 8192 -s 4096 /tmp/fsfile
fs created label mylabel on /tmp/fsfile
        nodesize 8192 leafsize 16384 sectorsize 4096 size 1.28GB
Btrfs v0.16-37-gb8271dc
# mkfs.xfs /tmp/fsfile
mkfs.xfs: /tmp/fsfile appears to contain an existing filesystem (btrfs).
mkfs.xfs: Use the -f option to force overwrite.

This'll fix up xfsqa failures when mkfs.btrfs is found, too.

Reviewed-by: Christoph Hellwig <hch@infradead.org>