KVM: MMU: remove rmap before clear spte

Remove rmap before clear spte otherwise it will trigger BUG_ON() in
some functions such as rmap_write_protect().

Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>