nilfs2: directory entry operations

This adds directory handling functions, most of which comes from the ext2
file system.

Signed-off-by: Ryusuke Konishi <[email protected]>
Signed-off-by: Yoshiji Amagai <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: Yoshiji Amagai