kcsan: Add current->state to implicitly atomic accesses

Add volatile current->state to list of implicitly atomic accesses. This
is in preparation to eventually enable KCSAN on kernel/sched (which
currently still has KCSAN_SANITIZE := n).

Since accesses that match the special check in atomic.h are rare, it
makes more sense to move this check to the slow-path, avoiding the
additional compare in the fast-path. With the microbenchmark, a speedup
of ~6% is measured.

Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>