ext4: Fix ext4_dx_readdir hash collision handling

This fixes a bug where readdir() would return a directory entry twice
if there was a hash collision in an hash tree indexed directory.

Signed-off-by: Eugene Dashevsky <eugene@ibrix.com>
Signed-off-by: Mike Snitzer <msnitzer@ibrix.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>