ext4: destroy ext4_es_cachep on module unload

Without this, module can't be reloaded.

[  500.521980] kmem_cache_sanity_check (ext4_extent_status): Cache name already exists.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: stable@vger.kernel.org  # v3.8+