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