common/rc: add missing 'local' keywords

common/rc: add missing 'local' keywords

Many helper functions in xfstests are unnecessarily declaring variables
without the 'local' keyword, which pollutes the global namespace and can
collide with variables in tests.  Fix this for everything in common/rc
that I could find.

In _math(), just eliminate $result by removing the check for nonempty
$BC, which is redundant with _require_math() which the tests do.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>