Revert "netfilter: xtables: compute exact size needed for jumpstack"

This reverts commit 98d1bd80.

mark_source_chains will not re-visit chains, so

*filter
:INPUT ACCEPT [365:25776]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [217:45832]
:t1 - [0:0]
:t2 - [0:0]
:t3 - [0:0]
:t4 - [0:0]
-A t1 -i lo -j t2
-A t2 -i lo -j t3
-A t3 -i lo -j t4
# -A INPUT -j t4
# -A INPUT -j t3
# -A INPUT -j t2
-A INPUT -j t1
COMMIT

Will compute a chain depth of 2 if the comments are removed.
Revert back to counting the number of chains for the time being.

Reported-by: Cong Wang <cwang@twopensource.com>
Reported-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>