Remove unionfs_islocked()

The implementation is racy; if the unionfs vnode is not in fact
locked, vnode private data may be concurrently altered or freed.
Instead, simply rely upon the standard implementation to query the
v_vnlock field, which is type-stable and will reflect the correct
lower/upper vnode configuration for the unionfs node.

Tested by: pho
Reviewed by: kib, markj
Differential Revision: https://reviews.freebsd.org/D39272