vm_kern: Avoid sign extension in the KVA_QUANTUM definition

Otherwise, on a powerpc64 NUMA system with hashed page tables, the
first-level superpage reservation size is large enough that the value of
the kernel KVA arena import quantum, KVA_NUMA_IMPORT_QUANTUM, is
negative and gets sign-extended when passed to vmem_set_import(). This
results in a boot-time hang on such platforms.

Approved by: re (gjb)
Reported by: bdragon

(cherry picked from commit 23e875fd97fb9f17b3f5dc2b26082f25e1a86b6f)
(cherry picked from commit 0486986ad81dbbfca291acf5b15f94d67d8a61bd)