KVM: MMU: drop zeroing on mmu_memory_cache_alloc

Zeroing on mmu_memory_cache_alloc is unnecessary since:

- Smaller areas are pre-allocated with kmem_cache_zalloc.
- Page pointed by ->spt is overwritten with prefetch_page
  and entries in page pointed by ->gfns are initialized
  before reading.

[avi: zeroing pages is unnecessary]

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>