JBD2: debug code cleanup.

Mostly stolen from akpm's JBD cleanup patch.

- use `#ifdef foo' instead of `#if defined(foo)'

- Make journal_enable_debug __read_mostly just for the heck of it

- Make jbd_debugfs_dir and jbd_debug static

- debugfs_remove(NULL) is legal: remove unneeded tests

- remove unnecessary empty loops

Signed-off-by: Jose R. Santos <jrs@us.ibm.com>
Cc: <linux-ext4@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>