kcsan: Move kcsan_{disable,enable}_current() to kcsan-checks.h

Both affect access checks, and should therefore be in kcsan-checks.h.
This is in preparation to use these in compiler.h.

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