netfilter: Fix slab corruption.

Use the correct pattern for singly linked list insertion and
deletion.  We can also calculate the list head outside of the
mutex.

Fixes: e3b37f11 ("netfilter: replace list_head with single linked list")
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Aaron Conole <aconole@bytheb.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/netfilter/core.c | 108 ++++++++++++++++-----------------------------------
 1 file changed, 33 insertions(+), 75 deletions(-)