btrfs-progs: introduce btrfs_register_all_device()

btrfs-progs: introduce btrfs_register_all_device()

This function is to register all devices found after scanning
the system. Before we had this functionality with in the
btrfs_scan_lblkid(), however scanning and registering are two
different distinct operation its better keep them separate.
Also we want to optimize btrfs_scan_lblkid and avoid multiple
system scans unless needed. As of now device scan uses this function.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.cz>