zfs: Fix vnode destruction on znode cache destruction

Looks like ZTOV(zp) couldn't be NULL at that time given that the value is passed to vn_free later on to free the underlying vnode.
For the purpose mentioned above, the statement 'zp->z_vnode = NULL' also needs to be moved around. Otherwise, ZTOV would always return NULL when zfs_znode_cache_destructor is reached.

Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>