Handle overflow in calculating max kmem size.

Handle overflow in calculating max kmem size.

vm_kmem_size is u_long, and it might be not capable of holding page
count times PAGE_SIZE, even when scaled down by VM_KMEM_SIZE_SCALE.  As
bde reported, 12G PAE config ends up with zero for kmem size.

Explicitly check for overflow and clamp kmem size at vm_kmem_size_max.
If we end up at zero size because VM_KMEM_SIZE_MAX is not defined,
panic with clear explanation rather then failing in a way which is
hard to relate.

Reported by:	bde, pho
Tested by:	pho
Reviewed by:	markj
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D18767