KVM: MMU: Introduce is_last_gpte() to clean up walk_addr_generic()

Suggested by Ingo and Avi.

Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>