fs: jbd2: make jbd2_journal_start() kernel-doc parseable

kernel-doc script expects that a function documentation to
be just before the function, otherwise it will be ignored.

So, move the kernel-doc markup to the right place.

Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>