jbd: jbd-debug and jbd2-debug should be writable

jbd-debug and jbd2-debug is currently read-only (S_IRUGO), which is not
correct. Make it writable so that we can start debuging.

Signed-off-by: Yin Kangkai <kangkai.yin@intel.com>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jan Kara <jack@suse.cz>