x86bios: use M_NOWAIT with mallocs

Or else it triggers the following bug:

APIC: CPU 6 has ACPI ID 6
APIC: CPU 7 has ACPI ID 7
panic: vm_wait in early boot
cpuid = 0
time = 1
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 0xffffffff826ff8d0
vpanic() at vpanic+0x1a3/frame 0xffffffff826ff930
panic() at panic+0x43/frame 0xffffffff826ff990
vm_wait_domain() at vm_wait_domain+0xf9/frame 0xffffffff826ff9c0
kmem_alloc_contig_domain() at kmem_alloc_contig_domain+0x252/frame 0xffffffff826ffa50
kmem_alloc_contig() at kmem_alloc_contig+0x6c/frame 0xffffffff826ffad0
contigmalloc() at contigmalloc+0x2e/frame 0xffffffff826ffb00
x86bios_modevent() at x86bios_modevent+0x225/frame 0xffffffff826ffb20
module_register_init() at module_register_init+0xc0/frame 0xffffffff826ffb50
mi_startup() at mi_startup+0x118/frame 0xffffffff826ffb70
start_kernel() at start_kernel+0x10

While there also make x86bios_unmap_mem idempotent.

Reviewed by: kib
Approved by: re (gjb)
Sponsored by: Citrix Systems R&D
Differential revision: https://reviews.freebsd.org/D17000