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

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>
(cherry picked from commit e9545b9f8aeb63e05818e4b3250057260bc072aa)
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>