pf: Rename pfsync bucket lock

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