vm: Use vmem_xalloc in kva_alloc

The kernel_arena used in kva_alloc has the qcache disabled. vmem_alloc
will first try to use the qcache before falling back to vmem_xalloc.

Rather than trying to use the qcache in vmem_alloc just call
vmem_xalloc directly.

Reviewed by: alc, kib, markj
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D42831

(cherry picked from commit 8daee410d2c13b4e8530b00e7877eeecf30bb064)