x86/platform/uv: Annotate struct uv_rtc_timer_head with __counted_by

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS
(for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

Found with Coccinelle:

  https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci

Add __counted_by for struct uv_rtc_timer_head.

Signed-off-by: Kees Cook <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Reviewed-by: Gustavo A. R. Silva <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: kees

arch/x86/platform/uv/uv_time.c

@@ -53,7 +53,7 @@ struct uv_rtc_timer_head {
 	struct {
 		int	lcpu;		/* systemwide logical cpu number */
 		u64	expires;	/* next timer expiration for this cpu */
-	} cpu[];
+	} cpu[] __counted_by(ncpus);
 };
 
 /*