btrfs: fix wrong accounting of raid1 data profile in statfs

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: ba7b6e62f420 ("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>
(cherry picked from commit 0d95c1bec906dd1ad951c9c001e798ca52baeb0f)