netfilter: conntrack: un-inline nf_ct_ecache_ext_add

Only called when new ct is allocated or the extension isn't present.
This function will be extended, place this in the conntrack module
instead of inlining.

The callers already depend on nf_conntrack module.
Return value is changed to bool, noone used the returned pointer.

Make sure that the core drops the newly allocated conntrack
if the extension is requested but can't be added.
This makes it necessary to ifdef the section, as the stub
always returns false we'd drop every new conntrack if the
the ecache extension is disabled in kconfig.

Add from data path (xt_CT, nft_ct) is unchanged.

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