ext4: wipe ext4_dir_entry2 upon file deletion

ext4: wipe ext4_dir_entry2 upon file deletion

Upon file deletion, zero out all fields in ext4_dir_entry2 besides rec_len.
In case sensitive data is stored in filenames, this ensures no potentially
sensitive data is left in the directory entry upon deletion. Also, wipe
these fields upon moving a directory entry during the conversion to an
htree and when splitting htree nodes.

The data wiped may still exist in the journal, but there are future
commits planned to address this.

Signed-off-by: Leah Rumancik <leah.rumancik@gmail.com>
Link: https://lore.kernel.org/r/20210422180834.2242353-1-leah.rumancik@gmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>