extensions: libebt_mark: Drop mark_supplied check

extensions: libebt_mark: Drop mark_supplied check

Use of this static variable causes trouble as it affects all instances
of this target. So calling xs_init_target() for one instance invalidates
all the others.

Moving the variable into target private data seems not possible since
that would change the target's size and therefore it wouldn't match
anymore with what kernel expects.

So just get rid of it entirely. If a user "forgets" to set a mark value,
the default value of zero applies.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>