jbd2: only create debugfs and stats entries if init is successful

jbd2 debugfs and stats entries should only be created if cache initialisation
is successful.  At the moment they are being created unconditionally which
will leave them dangling if cache (and hence module) initialisation fails.

Signed-off-by: Duane Griffin <duaneg@dghda.com>
Cc: <linux-ext4@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>