Increase a chance of devfs_close() calling d_close cdevsw method.

If a file opened over a vnode has an advisory lock set at close,
vn_closefile() acquires additional vnode use reference to prevent
freeing the vnode in vn_close(). Side effect is that for device
vnodes, devfs_close() sees that vnode reference count is greater than
one and refuses to call d_close(). Create internal version of
vn_close() which can avoid dropping the vnode reference if needed, and
use this to execute VOP_CLOSE() without acquiring a new reference.

Note that any parallel reference to the vnode would still prevent
d_close call, if the reference is not from an opened file, e.g. due to
stat(2).

Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks