netfilter: conntrack: proc: rename stat column

Rename 'searched' column to 'clashres' (same len).

conntrack(8) using the old /proc interface (ctnetlink not available) shows:

cpu=0  entries=4784 clashres=2292 [..]

Another alternative is to add another column, but this increases the
number of always-0 columns.

Fixes: bc924704 ("netfilter: conntrack: add clash resolution stat counter")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>