KVM: MMU: fix bogus alloc_mmu_pages assignment

Remove the bogus n_free_mmu_pages assignment from alloc_mmu_pages.

It breaks accounting of mmu pages, since n_free_mmu_pages is modified
but the real number of pages remains the same.

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