zfs: Fix znode reference count leaks

The zfs_remove() function calls zfs_dirent_lock, which in turn calls
zfs_zget() which bumps up the underlying znode reference count once.

However, neither zfs_remove() or zfs_rmdir() release the reference count
after using it. This prevents zfs_zinactive() which is used to destroy
the znode object from working properly. Another consequence is that each
znode holds a reference to the underlying mount point, keeping it busy
for unmount.

Fix the znode refcnt by calling zfs_zinactive after znode usage.

Reviewed-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>