vm_ksubmap_init: pass M_WAITOK to vmem_init -> uma_zalloc_arg

uma_zalloc_arg expects exactly one of the two WAIT flags. A future
commit will assert this.

Reviewed by: rstone
MFC after: 1 month
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D34450