ext3: improve some code in rb tree part of dir.c

- remove unnecessary code in free_rb_tree_fname
 - rename free_rb_tree_fname to ext3_htree_create_dir_info
   since it and ext3_htree_free_dir_info are a pair
 - replace kmalloc with kzalloc in ext3_htree_free_dir_info

Signed-off-by: Shen Feng <shen@cn.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>