jbd2: add transaction to checkpoint list earlier

We don't otherwise need j_list_lock during the rest of commit phase
#7, so add the transaction to the checkpoint list at the very end of
commit phase #6.  This allows us to drop j_list_lock earlier, which is
a good thing since it is a super hot lock.

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