Release the v_writecount reference on the vnode in case of error,

Release the v_writecount reference on the vnode in case of error,
before the vnode is vput() in vm_mmap_vnode().  Error return means
that there is no use reference on the vnode from the vm object
reference, and failing to restore v_writecount breaks the invariant
that v_writecount is less or equal to the usecount.

The situation observed when nfs client returns ESTALE for
VOP_GETATTR() after the open.

In collaboration with:	pho
MFC after:	1 week