bpf: Make bpf_peers_present a boolean inline function

This function was introduced in commit [1] and is actually used as a
boolean function although it was not defined as so.

No functional change intended.

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

Reviewed by: markj, kp, network
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D45509