Add MS_SILENT to most mount() invocations

There's an effort to migrate Linux filesystems to handle the y2038
problem, which is great.  However, recently a kernel change landed
that emits a warning when mounting a filesystem that doesn't
handle it, and this notably shows up even when *remounting* e.g.
for a read-only bind mount:

Using e.g. `rpm-ostree install cowsay` there's a spam of:

```
[  189.529594] xfs filesystem being remounted at /sysroot supports timestamps until 2038 (0x7fffffff)
```

Now particularly when creating a our bind mounts, let's
ask the kernel to be quiet about it.  This is not a major event
worthy of a kernel log.

bind-mount.c

@@ -389,7 +389,7 @@ bind_mount (int           proc_fd,
 
   if (src)
     {
-      if (mount (src, dest, NULL, MS_BIND | (recursive ? MS_REC : 0), NULL) != 0)
+      if (mount (src, dest, NULL, MS_SILENT | MS_BIND | (recursive ? MS_REC : 0), NULL) != 0)
         return 1;
     }
 
@@ -411,7 +411,7 @@ bind_mount (int           proc_fd,
   new_flags = current_flags | (devices ? 0 : MS_NODEV) | MS_NOSUID | (readonly ? MS_RDONLY : 0);
   if (new_flags != current_flags &&
       mount ("none", resolved_dest,
-             NULL, MS_BIND | MS_REMOUNT | new_flags, NULL) != 0)
+             NULL, MS_SILENT | MS_BIND | MS_REMOUNT | new_flags, NULL) != 0)
     return 3;
 
   /* We need to work around the fact that a bind mount does not apply the flags, so we need to manually
@@ -426,7 +426,7 @@ bind_mount (int           proc_fd,
           new_flags = current_flags | (devices ? 0 : MS_NODEV) | MS_NOSUID | (readonly ? MS_RDONLY : 0);
           if (new_flags != current_flags &&
               mount ("none", mount_tab[i].mountpoint,
-                     NULL, MS_BIND | MS_REMOUNT | new_flags, NULL) != 0)
+                     NULL, MS_SILENT | MS_BIND | MS_REMOUNT | new_flags, NULL) != 0)
             {
               /* If we can't read the mountpoint we can't remount it, but that should
                  be safe to ignore because its not something the user can access. */

bubblewrap.c

@@ -2632,7 +2632,7 @@ main (int    argc,
   /* Mark everything as slave, so that we still
    * receive mounts from the real root, but don't
    * propagate mounts to the real root. */
-  if (mount (NULL, "/", NULL, MS_SLAVE | MS_REC, NULL) < 0)
+  if (mount (NULL, "/", NULL, MS_SILENT | MS_SLAVE | MS_REC, NULL) < 0)
     die_with_error ("Failed to make / slave");
 
   /* Create a tmpfs which we will use as / in the namespace */
@@ -2655,7 +2655,7 @@ main (int    argc,
   if (mkdir ("newroot", 0755))
     die_with_error ("Creating newroot failed");
 
-  if (mount ("newroot", "newroot", NULL, MS_MGC_VAL | MS_BIND | MS_REC, NULL) < 0)
+  if (mount ("newroot", "newroot", NULL, MS_SILENT | MS_MGC_VAL | MS_BIND | MS_REC, NULL) < 0)
     die_with_error ("setting up newroot bind");
 
   if (mkdir ("oldroot", 0755))
@@ -2720,7 +2720,7 @@ main (int    argc,
   close_ops_fd ();
 
   /* The old root better be rprivate or we will send unmount events to the parent namespace */
-  if (mount ("oldroot", "oldroot", NULL, MS_REC | MS_PRIVATE, NULL) != 0)
+  if (mount ("oldroot", "oldroot", NULL, MS_SILENT | MS_REC | MS_PRIVATE, NULL) != 0)
     die_with_error ("Failed to make old root rprivate");
 
   if (umount2 ("oldroot", MNT_DETACH))