ext4: unregister features interface on module unload

commit 8f021222c1e2756ea4c9dde93b23e1d2a0a4ec37 upstream.

Ext4 features interface was not properly unregistered which led to
problems while unloading/reloading ext4 module. This commit fixes that by
adding proper kobject unregistration code into ext4_exit_fs() as well as
fail-path of ext4_init_fs()

Reported-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>