netfilter: ipset: drop logically empty buckets in mtype_del

netfilter: ipset: drop logically empty buckets in mtype_del

mtype_del() counts empty slots below n->pos in k, but it only drops the
bucket when both n->pos and k are zero. This misses buckets whose live
entries have all been removed while n->pos still points past deleted slots.

Treat a bucket as empty when all positions below n->pos are unused and
release it directly instead of shrinking it further.

Fixes: 8af1c6fbd923 ("netfilter: ipset: Fix forceadd evaluation path")
Cc: stable@vger.kernel.org
Reported-by: Juefei Pu <tomapufckgml@gmail.com>
Reported-by: Xin Liu <dstsmallbird@foxmail.com>
Signed-off-by: Yifan Wu <yifanwucs@gmail.com>
Co-developed-by: Yuan Tan <yuantan098@gmail.com>
Signed-off-by: Yuan Tan <yuantan098@gmail.com>
Reviewed-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>