Improve bufring impl:

- Remove unused br_prod_bufs member
- Fixup r241037: buf_ring pads br_prod_* and br_cons_* members at 128
  bytes, assuming a fixed cache line size for all the architectures.
  However, the above mentioned revision broke the padding.
  Use explicit padding to the CACHE_LINE_SIZE on the members that
  mark the initial new padded sections. Of course, the padding is not
  important for performance reasons in the DEBUG_BUFRING case, leaving
  br_cons members to share the cache line with br_lock.
- Fixup r244732: by removing incorrectly added membar in
  buf_ring_dequeue_sc() where surrounding locking shoud be enough.
- Drastically reduce the number of membar used (pratically reverting
  r244732) by switching rmb() in buf_ring_dequeue_mc() and wmb() in
  buf_ring_enqueue() to be complete barriers.  This, along with
  br_prod_bufs departure, should fix ordering issues as explained in
  the provided comments.

This patch is not targeted for MFC.

Sponsored by:	EMC / Isilon storage division
Reviewed by:	glebius

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