btrfs-progs: device scan use BTRFS_SCAN_LBLKID by default

btrfs-progs: device scan use BTRFS_SCAN_LBLKID by default

with this patch, BTRFS_SCAN_LBLKID (which leverages lblkid
to look for btrfs disks) would be the default scan method
to look for the btrfs disks. And thus the output as seen
in the latest btrfs fi show and btrfs fi show -m for the
mounted disks will have the consistent disks path.
(it was inconsistent (across disks) because btrfs dev scan
provided a different path from the mount command eg. below)

    devid    1 size 1.98GiB used 435.00MiB path /dev/mapper/mpatha
    devid    2 size 2.00GiB used 415.00MiB path /dev/dm-1

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