iptables: Fix memory leak when invoking xtables_find_match

iptables: Fix memory leak when invoking xtables_find_match

xtables_find_match() returns two different kind of pointers.
The first type is pointing to the function pointer table loaded
via dlopen(). The second type is a copy (via plain malloc) of the
first type. xtables_find_match() marks the copies/clones with
m == m->next. So we need to free the struct xtables_match which
point back to themself.

Also fix the issue that we didn't handle multple match instances
at the same time.

The memory leak is only visible when having more than one match
of a kind.