buf_ring/drbr: Add buf_ring_peek_clear_sc and use it in drbr_peek

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.

MFC after: 1 week
Sponsored by: Microsoft OSTC
Differential Revision: https://reviews.freebsd.org/D5416