vn_need_pageq_flush(): simplify

There is no need to own vnode interlock, since v_object is type stable
and can only change to/from NULL, and no other checks in the function
access fields protected by the interlock. Remove the need variable, the
result of the test is directly usable as return value.

Tested by: mav, pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week