In jbd2_get_transaction, a new transaction is initialized,
and set to the j_running_transaction. No need for a return
value, so remove it.

Also, adjust some comments to match the actual operation
of this function.
Signed-off-by: NLiu Song <liu.song11@zte.com.cn>
Signed-off-by: NTheodore Ts'o <tytso@mit.edu>
Reviewed-by: NJan Kara <jack@suse.cz>