rcu: merge fix for Convert ACCESS_ONCE() to READ_ONCE() and WRITE_ONCE()

This mirrors the change introduced by 7d0ae808 of same title
in Linus' tree; it's not obvious as a merge resolution since we moved
the function.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>