kcsan: Refactor reading of instrumented memory

Factor out the switch statement reading instrumented memory into a
helper read_instrumented_memory().

No functional change.

Signed-off-by: Marco Elver <elver@google.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>