In vn_mkdir(), use vrele() instead of vput() on the parent directory

In vn_mkdir(), use vrele() instead of vput() on the parent directory
vnode in the case that the target exists and is the same vnode as
the parent (i.e. "mkdir ."). The namei() call does not leave the
vnode locked in this case even though you might expect it to.

This bug was mostly harmless in practice because unlocking an already
unlocked vnode currently does not trigger any panics or warnings.

Reviewed by:	jeff