f2fs: fix extent cache memory leak

extent tree/node slab cache is created during f2fs insmod,
how, it isn't destroyed during f2fs rmmod, this patch fix
it by destroy extent tree/node slab cache once rmmod f2fs.

Signed-off-by: Wanpeng Li <[e-mail]>
Reviewed-by: Chao Yu <[e-mail]>
Signed-off-by: Jaegeuk Kim <[e-mail]>

fs/f2fs/super.c

···
 	remove_proc_entry("fs/f2fs", NULL);
 	f2fs_destroy_root_stats();
 	unregister_filesystem(&f2fs_fs_type);
+	destroy_extent_cache();
 	destroy_checkpoint_caches();
 	destroy_segment_manager_caches();
 	destroy_node_manager_caches();