btrfs: fix computation of max fs size for multiple device fs tests

btrfs: fix computation of max fs size for multiple device fs tests

We were sorting numerical values with the 'sort' tool without telling it
that we are sorting numbers, giving us unexpected ordering. So just pass
the '-n' option to the 'sort' tool.

Example:

$ echo -e "11\n9\n20" | sort
11
20
9

$ echo -e "11\n9\n20" | sort -n
9
11
20

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>