ANDROID: xt_quota2: remove trailing junk which might have a digit in it

Make sure string only contains the characters specified by userspace.

Fix cherry-picked from xtables-extensions project

Signed-off-by: Sam Liddicott <sam@liddicott.com>
Bug: 196046570
Test: passed netd test suites
Fixes: 10cda83a ("ANDROID: netfilter: xt_quota2: adding the
original quota2 from xtables-addons")
Signed-off-by: Todd Kjos <tkjos@google.com>
(cherry picked from https://git.code.sf.net/p/xtables-addons/xtables-addons
bc2bcc383c70b293bd816c29523a952ca8736fb5)
Change-Id: I965448564906e5fbf0fe6d6414f44d9e257ea195