KVM: async_pf: change async_pf_execute() to use get_user_pages(tsk => NULL)

async_pf_execute() passes tsk == current to gup(), this is doesn't
hurt but unnecessary and misleading. "tsk" is only used to account
the number of faults and current is the random workqueue thread.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Suggested-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/async_pf.c

@@ -81,7 +81,7 @@ static void async_pf_execute(struct work_struct *work)
 	might_sleep();
 
 	down_read(&mm->mmap_sem);
-	get_user_pages(current, mm, addr, 1, 1, 0, NULL, NULL);
+	get_user_pages(NULL, mm, addr, 1, 1, 0, NULL, NULL);
 	up_read(&mm->mmap_sem);
 	kvm_async_page_present_sync(vcpu, apf);