mm, mempool: disallow mempools based on slab caches with constructors

All occurrences of mempools based on slab caches with object constructors
have been removed from the tree, so disallow creating them.

We can only dereference mem->ctor in mm/mempool.c without including
mm/slab.h in include/linux/mempool.h.  So simply note the restriction,
just like the comment restricting usage of __GFP_ZERO, and warn on kernels
with CONFIG_DEBUG_VM() if such a mempool is allocated from.

We don't want to incur this check on every element allocation, so use
VM_BUG_ON().

Signed-off-by: David Rientjes <rientjes@google.com>
Cc: Dave Kleikamp <shaggy@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Sebastian Ott <sebott@linux.vnet.ibm.com>
Cc: Mikulas Patocka <mpatocka@redhat.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>