btrfs-progs: mkfs should be consistent in calling register device

btrfs-progs: mkfs should be consistent in calling register device

When we have one device we don't call register device.
(in fact not mandatory, but to make it consistent)
And when we have more than one we call register device.

reproducer:
Nothing in the kernel device list
cat /proc/fs/btrfs/devlist | egrep fsid | wc -l
0

mkfs.btrfs will automatically call register device when devices
is more than 1.

mkfs.btrfs -f /dev/sdb /dev/sdc
cat /proc/fs/btrfs/devlist | egrep fsid | wc -l
1

But it does not when there is only one device

mkfs.btrfs -f /dev/sdb
cat /proc/fs/btrfs/devlist | egrep fsid | wc -l
0

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