seqlock: Fix barriers
seqlock: Fix barriers

Turns out we didn't (and still don't) understand the C++11 memory
model and things were floating out of our supposedly read-seqlock
protected regions.  Put in some good, old fashion compile barriers to
make things right.

aclements/sv6,aclements/sv6,bowlofstew/sv6,bowlofstew/sv6,bowlofstew/sv6,bowlofstew/sv6,aclements/sv6,aclements/sv6,bowlofstew/sv6,aclements/sv6