bpf: remove redundant REG_LIVE_READ check in stacksafe()

stacksafe() is called in exact == NOT_EXACT mode only for states that
had been porcessed by clean_verifier_states(). The latter replaces
dead stack spills with a series of STACK_INVALID masks. Such masks are
already handled by stacksafe().

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250918-callchain-sensitive-liveness-v3-3-c3cd27bacc60@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: eddyz87

kernel/bpf/verifier.c

@@ -18785,13 +18785,6 @@ static bool stacksafe(struct bpf_verifier_env *env, struct bpf_func_state *old,
 		     cur->stack[spi].slot_type[i % BPF_REG_SIZE]))
 			return false;
 
-		if (!(old->stack[spi].spilled_ptr.live & REG_LIVE_READ)
-		    && exact == NOT_EXACT) {
-			i += BPF_REG_SIZE - 1;
-			/* explored state didn't use this */
-			continue;
-		}
-
 		if (old->stack[spi].slot_type[i % BPF_REG_SIZE] == STACK_INVALID)
 			continue;