KVM: MMU: Rename kvm_mmu_free_some_pages() to make_mmu_pages_available()

The current name "kvm_mmu_free_some_pages" should be used for something
that actually frees some shadow pages, as we expect from the name, but
what the function is doing is to make some, KVM_MIN_FREE_MMU_PAGES,
shadow pages available: it does nothing when there are enough.

This patch changes the name to reflect this meaning better; while doing
this renaming, the code in the wrapper function is inlined into the main
body since the whole function will be inlined into the only caller now.

Signed-off-by: Takuya Yoshikawa <yoshikawa_takuya_b1@lab.ntt.co.jp>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>