btrfs: fix wrong accounting of raid1 data profile in statfs

The sizes that are obtained from space infos are in raw units and have
to be adjusted according to the raid factor. This was missing for
f_bavail and df reported doubled size for raid1.

Reported-by: Martin Steigerwald <Martin@lichtvoll.de>
Fixes: ba7b6e62

 ("btrfs: adjust statfs calculations according to raid profiles")
CC: stable@vger.kernel.org
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>