(re)register_binfmt returns with -EBUSY

When a binary format is unregistered and re-registered, register_binfmt
fails with -EBUSY.  The reason is that unregister_binfmt does not set
fmt->next to NULL, and seeing (fmt->next != NULL), register_binfmt fails
with -EBUSY.

One can find his way around by explicitly setting fmt->next to NULL after
unregistering, but that is kind of unclean (one should better be using only
the interfaces, and not the interal members, isn't it?)

Attached one-liner can fix it.
Signed-off-by: Kalash Nainwal <kalash.nainwal@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/exec.c

@@ -100,6 +100,7 @@ int unregister_binfmt(struct linux_binfmt * fmt)
 	while (*tmp) {
 		if (fmt == *tmp) {
 			*tmp = fmt->next;
+			fmt->next = NULL;
 			write_unlock(&binfmt_lock);
 			return 0;
 		}