mount --bind accounting

Something entirely different.

Last year I added the comment
	/* No capabilities? What if users do thousands of these? */
in super.c for "mount --bind".
Now that I do some polishing there I noticed the comment again.

Each bind does an alloc_vfsmnt() and hence takes some kernel memory.
Any user can therefore take all kernel memory, until
	kmalloc(sizeof(struct vfsmount), GFP_KERNEL)
fails. Bad security.
I suppose something needs to be done about that.

Invent an arbitrary limit
	#define MAX_NUMBER_OF_USER_BINDS 666
and refuse the "mount --bind" when inspection of the mnt_owner fields
of the entries in the vfsmntlist shows that this user already has
too many mounts?

Andries