KVM: MMU: Fix ubsan warnings

kvm_mmu_pages_init is doing some really yucky stuff.  It is setting
up a sentinel for mmu_page_clear_parents; however, because of a) the
way levels are numbered starting from 1 and b) the way mmu_page_path
sizes its arrays with PT64_ROOT_LEVEL-1 elements, the access can be
out of bounds.  This is harmless because the code overwrites up to the
first two elements of parents->idx and these are initialized, and
because the sentinel is not needed in this case---mmu_page_clear_parents
exits anyway when it gets to the end of the array.  However ubsan
complains, and everyone else should too.

This fix does three things.  First it makes the mmu_page_path arrays
PT64_ROOT_LEVEL elements in size, so that we can write to them without
checking the level in advance.  Second it disintegrates kvm_mmu_pages_init
between mmu_unsync_walk (to reset the struct kvm_mmu_pages) and
for_each_sp (to place the NULL sentinel at the end of the current path).
This is okay because the mmu_page_path is only used in
mmu_pages_clear_parents; mmu_pages_clear_parents itself is called within
a for_each_sp iterator, and hence always after a call to mmu_pages_next.
Third it changes mmu_pages_clear_parents to just use the sentinel to
stop iteration, without checking the bounds on level.

Reported-by: Sasha Levin <sasha.levin@oracle.com>
Reported-by: Mike Krinkin <krinkin.m.u@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>