nspawn: create boot_id and kmsg files for overmounting in /run, not /tmp

/tmp might not be mounted at all yet (given that we support
SYSTEMD_NSPAWN_TMPFS_TMP=0 to turn this off), and /tmp is a dir systemd
usually tries to unmount during shutdown (unlike /run), and we shouldn't
keep it busy. Hence let's just move these deleted files to /run so that
we don't keep /tmp needlessly busy.

src/nspawn/nspawn.c

@@ -1849,10 +1849,9 @@ static int setup_boot_id(void) {
         const char *to;
         int r;
 
-        /* Generate a new randomized boot ID, so that each boot-up of
-         * the container gets a new one */
+        /* Generate a new randomized boot ID, so that each boot-up of the container gets a new one */
 
-        r = tempfn_random_child(NULL, "proc-sys-kernel-random-boot-id", &path);
+        r = tempfn_random_child("/run", "proc-sys-kernel-random-boot-id", &path);
         if (r < 0)
                 return log_error_errno(r, "Failed to generate random boot ID path: %m");
 
@@ -2096,19 +2095,18 @@ static int setup_kmsg(int kmsg_socket) {
         _cleanup_free_ char *fifo = NULL;
         _cleanup_close_ int fd = -1;
         _cleanup_umask_ mode_t u;
-        const char *to;
         int r;
 
         assert(kmsg_socket >= 0);
 
         u = umask(0000);
 
-        /* We create the kmsg FIFO as as temporary file in /tmp, but immediately delete it after bind mounting it to
+        /* We create the kmsg FIFO as as temporary file in /run, but immediately delete it after bind mounting it to
          * /proc/kmsg. While FIFOs on the reading side behave very similar to /proc/kmsg, their writing side behaves
          * differently from /dev/kmsg in that writing blocks when nothing is reading. In order to avoid any problems
          * with containers deadlocking due to this we simply make /dev/kmsg unavailable to the container. */
 
-        r = tempfn_random_child(NULL, "proc-kmsg", &fifo);
+        r = tempfn_random_child("/run", "proc-kmsg", &fifo);
         if (r < 0)
                 return log_error_errno(r, "Failed to generate kmsg path: %m");
 
@@ -2116,9 +2114,8 @@ static int setup_kmsg(int kmsg_socket) {
                 return log_error_errno(errno, "mkfifo() for /run/kmsg failed: %m");
 
         from = TAKE_PTR(fifo);
-        to = "/proc/kmsg";
 
-        r = mount_verbose(LOG_ERR, from, to, NULL, MS_BIND, NULL);
+        r = mount_verbose(LOG_ERR, from, "/proc/kmsg", NULL, MS_BIND, NULL);
         if (r < 0)
                 return r;