ext4: rename "dirent_csum" functions to use "dirblock"
Functions such as ext4_dirent_csum_verify() and ext4_dirent_csum_set()
don't actually operate on a directory entry, but a directory block.
And while they take a struct ext4_dir_entry *dirent as an argument, it
had better be the first directory at the beginning of the direct
block, or things will go very wrong.
Rename the following functions so that things make more sense, and
remove a lot of confusing casts along the way: