systemctl: avoid spurious warning about missing reboot-param file

$ reboot -f
Failed to read reboot parameter file: No such file or directory

It seems that the warning on ENOENT was inadvertently introduced in
27c06cb.

The warning reported in systemd#5646 comes from systemctl, but let's fix the other
call site in the same way too.

Fixes systemd#5646.

Author: keszybz

src/core/shutdown.c

@@ -403,7 +403,7 @@ int main(int argc, char *argv[]) {
                         _cleanup_free_ char *param = NULL;
 
                         r = read_one_line_file("/run/systemd/reboot-param", &param);
-                        if (r < 0)
+                        if (r < 0 && r != -ENOENT)
                                 log_warning_errno(r, "Failed to read reboot parameter file: %m");
 
                         if (!isempty(param)) {

src/systemctl/systemctl.c

@@ -8260,7 +8260,7 @@ static int halt_now(enum action a) {
                 _cleanup_free_ char *param = NULL;
 
                 r = read_one_line_file("/run/systemd/reboot-param", &param);
-                if (r < 0)
+                if (r < 0 && r != -ENOENT)
                         log_warning_errno(r, "Failed to read reboot parameter file: %m");
 
                 if (!isempty(param)) {