ext4 crypto: simplify and speed up filename encryption

Avoid using SHA-1 when calculating the user-visible filename when the
encryption key is available, and avoid decrypting lots of filenames
when searching for a directory entry in a directory block.

Change-Id: If4655f144784978ba0305b597bfa1c8d7bb69e63
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
diff --git a/fs/ext4/dir.c b/fs/ext4/dir.c
index 61db51a5..5665d82 100644
--- a/fs/ext4/dir.c
+++ b/fs/ext4/dir.c
@@ -249,7 +249,7 @@
 				} else {
 					/* Directory is encrypted */
 					err = ext4_fname_disk_to_usr(enc_ctx,
-							de, &fname_crypto_str);
+						NULL, de, &fname_crypto_str);
 					if (err < 0)
 						goto errout;
 					if (!dir_emit(ctx,