secmark: do not return early if there was no error

Commit 4a5a5c73 attempted to pass decent error messages back to userspace for
netfilter errors.  In xt_SECMARK.c however the patch screwed up and returned
on 0 (aka no error) early and didn't finish setting up secmark.  This results
in a kernel BUG if you use SECMARK.

Signed-off-by: Eric Paris <eparis@redhat.com>
Acked-by: Paul Moore <paul.moore@hp.com>
Signed-off-by: James Morris <jmorris@namei.org>