vm_ksubmap_init: pass M_WAITOK to vmem_init -> uma_zalloc_arg

[vangyzen]

Summary

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

MFC after: 1 month
Sponsored by: Dell EMC Isilon

Test Plan

The whole kyua suite completed without a panic or a stack-dump like this (see D34452):

uma_zalloc_arg called with bad WAIT flags:
#0 0xffffffff80f72ca3 at uma_zalloc_arg+0x1a3
#1 0xffffffff80c794a0 at _bt_fill+0x100
#2 0xffffffff80c765a3 at vmem_init+0x373
#3 0xffffffff80f8010c at vm_ksubmap_init+0x13c
#4 0xffffffff810c560c at cpu_startup+0x20c
#5 0xffffffff80b91a9f at mi_startup+0x20f
#6 0xffffffff80389022 at btext+0x22