spawn-unix: Don't log an error if unable to reset Linux OOM score

We cannot safely log between fork() and exec() because it isn't an
async-signal-safe operation (in particular it might allocate memory).

We also don't want to treat a failure here as a real problem, because
it might legitimately not work: in a system dbus-daemon that has dropped
privileges from root, the pseudo-file representing this process parameter
remains owned by root and cannot be altered by the unprivileged user.

For the main use-case for this operation, the system dbus-daemon, we
have another opportunity to do this in the dbus-daemon-launch-helper
(see the previous commit).

Signed-off-by: Simon McVittie <smcv@collabora.com>
(cherry picked from commit 7ee72a2795)

dbus/dbus-spawn.c

@@ -1397,13 +1397,11 @@ _dbus_spawn_async_with_babysitter (DBusBabysitter          **sitter_p,
 	}
       else if (grandchild_pid == 0)
         {
-          const char *error_str = NULL;
-
-          if (!_dbus_reset_oom_score_adj (&error_str))
-            {
-              /* TODO: Strictly speaking, this is not async-signal-safe. */
-              _dbus_warn ("%s: %s", error_str, strerror (errno));
-            }
+          /* This might not succeed in a dbus-daemon that started as root
+           * and dropped privileges, so don't log an error on failure.
+           * (Also, we can't safely log errors here anyway, because logging
+           * is not async-signal safe). */
+          _dbus_reset_oom_score_adj (NULL);
 
           /* Go back to ignoring SIGPIPE, since it's evil
            */