vn_open_vnode(): handle error when fp == NULL

vn_open_vnode(): handle error when fp == NULL

If VOP_ADD_WRITECOUNT() or adv locking failed, so VOP_CLOSE() needs to
be called, we cannot use fp fo_close() when there is no fp.  This occurs
when e.g. kernel code directly calls vn_open() instead of the open(2)
syscall.

In this case, VOP_CLOSE() can be called directly, after possible lock
upgrade.

Reported by:	nvass@gmx.com
PR:	255119
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D29830