jbd2: use NULL instead of 0 when pointer is needed

Fixes sparse warning:

fs/jbd2/journal.c:1892:9: warning: Using plain integer as NULL pointer

Signed-off-by: Bill Pemberton <wfp5p@virginia.edu>
CC: linux-ext4@vger.kernel.org
Signed-off-by: Jiri Kosina <jkosina@suse.cz>