pfsync: Correctly check if bpf peers are present

[zlei]

Summary

On creating the pfsync(4) interface, pfsync_clone_create() does an
unconditional bpfattach(). Use bpf_peers_present() which was introduced
in commit 16d878cc99ef [1] to check the presence of bpf peers.

This will save a little CPU cycles and memory usage when the synchronisation interface
is not configured and there is no bpf peers present. There should be no functional change.

1. 16d878cc99ef Fix the following bpf(4) race condition which can result in a panic

MFC after: 1 week