KVM: MMU: increase per-vcpu rmap cache alloc size

KVM: MMU: increase per-vcpu rmap cache alloc size

(cherry picked from commit c41ef344de212bd918f7765af21b5008628c03e0)

The page fault path can use two rmap_desc structures, if:

- walk_addr's dirty pte update allocates one rmap_desc.
- mmu_lock is dropped, sptes are zapped resulting in rmap_desc being
  freed.
- fetch->mmu_set_spte allocates another rmap_desc.

Increase to 4 for safety.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>