buf_ring: Remove unneeded memory barriers

We no longer need a memory barrier on the compare and set operations.
If these fail then there are appropriate barriers on the loads earlier
in the loop, and if they succeed then no other threads can be accessing
the br_ring entry so any store to it is safe.

Reviewed by: alc
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D46381