[NETFILTER]: return ENOMEM when ip_conntrack_alloc() fails.

This patch fixes the bug which doesn't return ERR_PTR(-ENOMEM) if it
failed to allocate memory space from slab cache.  This bug leads to
erroneously not dropped packets under stress, and wrong statistic
counters ('invalid' is incremented instead of 'drop').  It was
introduced during the ctnetlink merge in the net-2.6.14 tree, so no
stable or mainline releases affected.

Signed-off-by: Yasuyuki Kozakai <yasuyuki.kozakai@toshiba.co.jp>
Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>