btrfs-progs: congregate dev scan

btrfs-progs: congregate dev scan

the dev scan to find btrfs is performed at two locations
all most the same way one at filesystem show and another
at device scan. They both follow the same steps. This
patch does not alter anything except that it brings these
two same logic into the function scan_for_btrfs so that
we can play tweaking it.

the patch which recommends to use /dev/mapper
will also need it

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>