KVM: MMU: cleanup for function unaccount_shadowed()

Since gfn is not changed in the for loop, we do not need to call
gfn_to_memslot_unaliased() under the loop, and it is safe to move
it out.

Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>
Signed-off-by: Avi Kivity <avi@redhat.com>