bpf: ensure speculations drops are reported when needed

The unrolled loop in dt_speculation() would return without recording
a speculation drop if an iteration was attempted beyond the configured
NSPEC value.  Depending on how the compiler optimized the code, that
could lead to an early return from the function without ever calling
dt_no_spec().

Signed-off-by: Kris Van Hees <kris.van.hees@oracle.com>
Reviewed-by: Nick Alcock <nick.alcock@oracle.com>
Reviewed-by: Eugene Loh <eugene.loh@oracle.com>

bpf/speculation.c

@@ -54,7 +54,7 @@ noinline uint32_t dt_speculation(void)
 #define SEARCH(n)							\
 	do {								\
 		if (n > (uint64_t) &NSPEC)				\
-			return 0;					\
+			goto no_spec;					\
 		id = (n);						\
 		if (bpf_map_update_elem(&specs, &id, &zero,		\
 			BPF_NOEXIST) == 0)				\
@@ -99,6 +99,7 @@ noinline uint32_t dt_speculation(void)
 	}
 #endif
 
+no_spec:
 	return dt_no_spec(busy ? DT_STATE_SPEC_BUSY : DT_STATE_SPEC_UNAVAIL);
 }