core: make mount(8) and swapon(8) inherit SMACK label from systemd

By default mount(8), umount(8), swapon(8) and swapoff(8) should run with
with the SMACK label inherited from systemd rather than the default one
meant for services.

Fixes: aa5ae9711e
Follow-up-for: 20bbf5ee4c

src/core/mount.c

@@ -876,6 +876,9 @@ static int mount_spawn(Mount *m, ExecCommand *c, PidRef *ret_pid) {
         if (r < 0)
                 return r;
 
+        /* Assume the label inherited from systemd as the fallback */
+        exec_params.fallback_smack_process_label = NULL;
+
         r = exec_spawn(UNIT(m),
                        c,
                        &m->exec_context,

src/core/swap.c

@@ -642,6 +642,9 @@ static int swap_spawn(Swap *s, ExecCommand *c, PidRef *ret_pid) {
         if (r < 0)
                 return r;
 
+        /* Assume the label inherited from systemd as the fallback */
+        exec_params.fallback_smack_process_label = NULL;
+
         r = exec_spawn(UNIT(s),
                        c,
                        &s->exec_context,