Fix the assertions for the state of the object under the map entry
with the MAP_ENTRY_VN_WRITECNT flag:
- Move the assertion that verifies the state of the v_writecount and
  vnp.writecount, under the block where the object is locked.
- Check that the object type is OBJT_VNODE before asserting.

Reported by:	avg
Reviewed by:	alc
MFC after:	1 week