ucode-mod-bpf: fix error reporting in tc hook setup

Failures in the detach path returned NULL without recording any error
state, so error() reported a stale or empty error. Attach failures
reported a hardcoded ENOENT regardless of the actual cause. Record
errno for all failures; if_nametoindex, bpf_tc_attach and
bpf_tc_detach all set it.

Signed-off-by: Felix Fietkau <nbd@nbd.name>

package/utils/ucode-mod-bpf/src/bpf.c

@@ -680,9 +680,7 @@ out:
 	return TRUE;
 
 error:
-	if (fd >= 0)
-		err_return(ENOENT, NULL);
-	return NULL;
+	err_return(errno, NULL);
 }
 
 static uc_value_t *