btrfs-progs: Fix init script

the init script calls the old btrfsctl util, which is not included in
btrfs-progs anymore. Update the init script to call "btrfs device scan"
which assembles multi device btrfs filesystems.

Signed-off-by: Stefan Hellermann <stefan@the2masters.de>
Merge pull request #612 from the2masters/btrfs-progs-init-script

btrfs-progs: Fix init script