buf_ring/drbr: Add buf_ring_peek_clear_sc and use it in drbr_peek

[sepherosa_gmail.com]

Summary

Unlike buf_ring_peek, it only supports single consumer mode, and it clears the cons_head if DEBUG_BUFRING/INVARIANTS is defined.

The normal use case of drbr_peek for network drivers is:

m = drbr_peek(br);
err = hw_spec_encap(&m); /* could m_defrag/m_collapse */
(*)
if (err) {
    if (m == NULL)
        drbr_advance(br);
    else
        drbr_putback(br, m);
    /* break the loop */
}
drbr_advance(br);

The race is:
If hw_spec_encap() m_defrag or m_collapse the mbuf, i.e. the old mbuf was freed, or like the Hyper-V's network driver, that transmission-done does not even require the TX lock; then on the other CPU at the (*) time, the freed mbuf could be recycled and being drbr_enqueue even before the current CPU had the chance to call drbr_{advance,putback}. This triggers a panic in drbr_enqueue duplicated element check, if DEBUG_BUFRING/INVARIANTS is defined.

Use buf_ring_peek_clear_sc() in drbr_peek() to fix the above race.

This change is a NO-OP, if neither DEBUG_BUFRING nor INVARIANTS are defined.

[sepherosa_gmail.com]

It will be committed next Monday if no objection comes.