pmap: Keep PTI page table pages busy

PTI page table pages are allocated from a VM object, so must be
exclusively busied when they are freed, e.g., when a thread loses a race
in pmap_pti_pde(). Simply keep PTPs busy at all times, as was done for
some other kernel allocators in commit
e9ceb9dd110e04fc19729b4e9fb1c8bfbb8398a3.

Also remove some redundant assertions on "ref_count":
vm_page_unwire_noq() already asserts that the page's reference count is
greater than zero.

Reported by: syzkaller
Reviewed by: kib
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35466