mnl: fix --echo buffer size again

mnl: fix --echo buffer size again

If restart is triggered with --echo, it causes rules to be duplicated
which is not correct. Remove restart logic.

1. If user passes --echo, use a default 4mb buffer.
2. assume each element in the batch will result in a 1k
   notification.

This passes tests both in x86_64 and s390.

Joint work with Florian Westphal.

Fixes: 877baf9538f6 ("src: mnl: retry when we hit -ENOBUFS")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>