bpf: Improve ctx access verifier error message

We've already had two "error during ctx access conversion" warnings
triggered by syzkaller. Let's improve the error message by dumping the
cnt variable so that we can more easily differentiate between the
different error cases.

Signed-off-by: Paul Chaignon <[email protected]>
Acked-by: Eduard Zingerman <[email protected]>
Link: https://lore.kernel.org/r/cc94316c30dd76fae4a75a664b61a2dbfe68e205.1754039605.git.paul.chaignon@gmail.com
Signed-off-by: Alexei Starovoitov <[email protected]>

Author: pchaigno

kernel/bpf/verifier.c

@@ -21445,7 +21445,7 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
 					 &target_size);
 		if (cnt == 0 || cnt >= INSN_BUF_SIZE ||
 		    (ctx_field_size && !target_size)) {
-			verifier_bug(env, "error during ctx access conversion");
+			verifier_bug(env, "error during ctx access conversion (%d)", cnt);
 			return -EFAULT;
 		}