netfilter: conntrack: handle initial extension alloc via krealloc

krealloc(NULL, ..) is same as kmalloc(), so we can avoid special-casing
the initial allocation after the prealloc removal (we had to use
->alloc_len as the initial allocation size).

This also means we do not zero the preallocated memory anymore; only
offsets[].  Existing code makes sure the new (used) extension space gets
zeroed out.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>