CVE-2026-98794: ipvs: filter some flags received in the backup server

In the Linux kernel, the following vulnerability has been resolved:

ipvs: filter some flags received in the backup server

While the IPVS SYNC protocol is not secure by design we can still protect the backup server from messages that can wreak havoc.

This commit addresses problems from received connection flags or their combinations. We now drop messages as follows:

1. the NOCPORT+TEMPLATE combination allows lookups for normal connections to hit template which can break in many ways. While the master does not sync connections with NOCPORT flag, i.e. before they are established, we still accept NOCPORT without TEMPLATE.

2. ONEPACKET: it is not sent by master, so we do not expect it in backup. Before now it was ignored by IPVSCONNFBACKUPMASK for protocol v1 while protocol v0 created connections that are not hashed and dropped immediately. Better to apply the IPVSCONNFBACKUPMASK also to the flags from v0 messages for consistency with v1.