ext4: improve some code in rb tree part of dir.c
ext4: improve some code in rb tree part of dir.c

* remove unnecessary code in free_rb_tree_fname

* rename free_rb_tree_fname to ext4_htree_create_dir_info
  since it and ext4_htree_free_dir_info are a pair

* replace kmalloc with kzalloc in ext4_htree_free_dir_info

All these make the code more readable and simple.
PS: this patch is also suitable for ext3.

Signed-off-by: Shen Feng <ba51191cd9d28890339dabfae3100b9d0121a68d@cn.fujitsu.com>
Signed-off-by: Mingming Cao <89e10773c335857ac236dfae53fd89ae361ca3d4@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <4ed386e0495d3e109932df055831d9ec2f824927@mit.edu>

KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas