arm64: hw_breakpoint: Fix warnings about implicit fallthrough

Now that -Wimplicit-fallthrough is passed to GCC by default, the kernel
build has suddenly got noisy. Annotate the two fall-through cases in our
hw_breakpoint implementation, since they are both intentional.
Reported-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/kernel/hw_breakpoint.c

@@ -540,10 +540,14 @@ int hw_breakpoint_arch_parse(struct perf_event *bp,
 			/* Allow halfword watchpoints and breakpoints. */
 			if (hw->ctrl.len == ARM_BREAKPOINT_LEN_2)
 				break;
+
+			/* Fallthrough */
 		case 3:
 			/* Allow single byte watchpoint. */
 			if (hw->ctrl.len == ARM_BREAKPOINT_LEN_1)
 				break;
+
+			/* Fallthrough */
 		default:
 			return -EINVAL;
 		}