Avoid the possibility of integer overflow in the calculation of VM_KMEM_SIZE_MAX.  Specifically, if the user/kernel address space split was changed such that the kernel address space was greater than or equal to 2 GB, then overflow would occur.
Avoid the possibility of integer overflow in the calculation of
VM_KMEM_SIZE_MAX.  Specifically, if the user/kernel address space split
was changed such that the kernel address space was greater than or equal
to 2 GB, then overflow would occur.

PR:		161721
MFC after:	3 weeks

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase