wg: detach bpf upon destroy as well

bpfattach() is called in wg_clone_create(), but the bpfdetach() is
missing from wg_close_destroy(). Add the missing bpfdetach() to avoid
leaking both the associated bpf bits as well as the ifnet that bpf will
hold a reference to.

PR: 276526
MFC after: 3 days