jbd2: clean up an assertion in jbd2_journal_commit_transaction()

commit_transaction has the same value as journal->j_running_transaction,
so we can simplify the assert statement.

Signed-off-by: dingdinghua <dingdinghua@nrchpc.ac.cn>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>