tree-wide: fix spelling of "fallback"

Similarly to "setup" vs. "set up", "fallback" is a noun, and "fall back"
is the verb. (This is pretty clear when we construct a sentence in the
present continous: "we are falling back" not "we are fallbacking").

src/core/manager.c

@@ -4371,11 +4371,11 @@ const char *manager_get_confirm_spawn(Manager *m) {
          *
          * If the console suddenly disappear at the time our children will really it
          * then they will simply fail to acquire it and a positive answer will be
-         * assumed. New children will fallback to /dev/console though.
+         * assumed. New children will fall back to /dev/console though.
          *
          * Note: TTYs are devices that can come and go any time, and frequently aren't
          * available yet during early boot (consider a USB rs232 dongle...). If for any
-         * reason the configured console is not ready, we fallback to the default
+         * reason the configured console is not ready, we fall back to the default
          * console. */
 
         if (!m->confirm_spawn || path_equal(m->confirm_spawn, "/dev/console"))

src/core/namespace.c

@@ -648,7 +648,7 @@ static int clone_device_node(
                 *make_devnode = false;
         }
 
-        /* We're about to fallback to bind-mounting the device
+        /* We're about to fall back to bind-mounting the device
          * node. So create a dummy bind-mount target.
          * Do not prepare device-node SELinux label (see issue 13762) */
         r = mknod(dn, S_IFREG, 0);

src/core/service.c

@@ -1034,7 +1034,7 @@ static void service_search_main_pid(Service *s) {
 
         assert(s);
 
-        /* If we know it anyway, don't ever fallback to unreliable
+        /* If we know it anyway, don't ever fall back to unreliable
          * heuristics */
         if (s->main_pid_known)
                 return;