pf: Rename pfsync bucket lock

Previously the main pfsync lock and the bucket locks shared the same name.
This lead to spurious warnings from WITNESS like this:

acquiring duplicate lock of same type: "pfsync"
 1st pfsync @ /usr/src/sys/netpfil/pf/if_pfsync.c:1402
 2nd pfsync @ /usr/src/sys/netpfil/pf/if_pfsync.c:1429

It's perfectly okay to grab both the main pfsync lock and a bucket lock at the
same time.

We don't need different names for each bucket lock, because we should always
only acquire a single one of those at a time.

MFC after: 1 week