shutdown: unregister all binfmt_misc entries before entering shutdown loop

Apparently if the new "F" flag is used they might pin files, which
blocks us from unmounting things. Let's hence clear this up explicitly.
Before entering our umount loop.

Fixes: #14981

src/shutdown/shutdown.c

@@ -16,6 +16,7 @@
 
 #include "alloc-util.h"
 #include "async.h"
+#include "binfmt-util.h"
 #include "cgroup-setup.h"
 #include "cgroup-util.h"
 #include "def.h"
@@ -386,6 +387,7 @@ int main(int argc, char *argv[]) {
                 sync_with_progress();
 
         disable_coredumps();
+        disable_binfmt();
 
         log_info("Sending SIGTERM to remaining processes...");
         broadcast_signal(SIGTERM, true, true, arg_timeout);