btrfs-progs: scan /proc/partitions not all of /dev with "-d"

btrfs-progs: scan /proc/partitions not all of /dev with "-d"

We can scan for btrfs devices in a few ways.  By default
libblkid is used for "device scan" and "filesystem show";
with the -m option only mounted filesystems are scanned,
and with -d we physically read every system device.

But there's no reason for the complexity of a descent through
/dev; /proc/partitions has every device known to the kernel, so
just use that when -d is specified.

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