is_same_fs() in lib/archroot.sh should use $1 and $2, not $1 and $1

The old behaviour would always evaluate to true - this is certainly not
what that function should do.

Signed-off-by: Erich Eckner <git@eckner.net>

lib/archroot.sh

@@ -42,7 +42,7 @@ is_subvolume() {
 # return : whether $path_a and $path_b are on the same filesystem
 ##
 is_same_fs() {
-	[[ "$(stat -c %d "$1")" == "$(stat -c %d "$1")" ]]
+	[[ "$(stat -c %d "$1")" == "$(stat -c %d "$2")" ]]
 }
 
 ##