libblkid-tiny: add btrfs probe info

This only adds the magic for btrfs to show btrfs volumes in
block detect.

In order to properly support btrfs with more than one device and avoid
trying to mount the same device group once for each instance we have to
make sure that only one device per group will be mounted (btrfs then
automatically detects all other devices of the group).
For unmounting, the opposite should be applied: umount if called for
any of the devices in a mounted device group, unmount the filesystem.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Felix Fietkau <nbd@openwrt.org>