ext4 crypto: teach ext4_htree_store_dirent() to store decrypted filenames

For encrypted directories, we need to pass in a separate parameter for
the decrypted filename, since the directory entry contains the
encrypted filename.

Change-Id: I7290eb0979d9e9d9d65ee07bbf11223b3382394f
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Theodore Ts'o <tytso@google.com>