Avoid sign extension of value passed to kva_alloc from uma_zone_reserve_kva

Fixes "panic: vm_radix_reserve_kva: unable to reserve KVA" caused by sign
extention of "pages * UMA_SLAB_SIZE" value passed to kva_alloc() which
takes unsigned long argument.

In the erroneus case that triggered this bug, the number of pages
to allocate in uma_zone_reserve_kva() was 0x8ebe6, that gave the
total number of bytes to allocate equal to 0x8ebe6000 (int).
This was then sign extended in kva_alloc() to 0xffffffff8ebe6000
(unsigned long).

Reviewed by: alc, kib
Submitted by: Zbigniew Bodek <zbb@semihalf.com>
Obtained from: Semihalf
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D3346