tracing/probes: Avoid using params uninitialized in parse_btf_arg()

After a recent change in clang to strengthen uninitialized warnings [1],
it points out that in one of the error paths in parse_btf_arg(), params
is used uninitialized:

  kernel/trace/trace_probe.c:660:19: warning: variable 'params' is uninitialized when used here [-Wuninitialized]
    660 |                         return PTR_ERR(params);
        |                                        ^~~~~~

Match many other NO_BTF_ENTRY error cases and return -ENOENT, clearing
up the warning.

Link: https://lore.kernel.org/all/20250715-trace_probe-fix-const-uninit-warning-v1-1-98960f91dd04@kernel.org/

Cc: [email protected]
Closes: ClangBuiltLinux/linux#2110
Fixes: d157d76 ("tracing/probes: Support BTF field access from $retval")
Link: llvm/llvm-project@2464313 [1]
Signed-off-by: Nathan Chancellor <[email protected]>
Signed-off-by: Masami Hiramatsu (Google) <[email protected]>

Author: nathanchance

kernel/trace/trace_probe.c

@@ -657,7 +657,7 @@ static int parse_btf_arg(char *varname,
 		ret = query_btf_context(ctx);
 		if (ret < 0 || ctx->nr_params == 0) {
 			trace_probe_log_err(ctx->offset, NO_BTF_ENTRY);
-			return PTR_ERR(params);
+			return -ENOENT;
 		}
 	}
 	params = ctx->params;