From: Alex Tomas <bzzz@tmi.comex.ru>

Some transaction checkpointing improvements for the JBD commit phase.  Decent
speedups:

creation of 500K files in single dir (with htree, of course):
 before: 4m16.094s, 4m12.035s, 4m11.911s
 after:  1m41.364s, 1m43.461s, 1m45.189s

removal of 500K files in single dir:
 before: 43m50.161s
 after:  38m45.510s


- Make __log_wait_for_space() recalculate the needed blocks because journal
  free space changes during commit

- Make log_do_checkpoint() starts scanning from the oldest transaction

- Make log_do_checkpoint() stop scanning if a transaction gets dropped.
  The caller will reevaluate the transaction state and decide whether more
  space needs to be generated in the log.

  The effect of this is to smooth out the I/O patterns, avoid the huge
  stop-and-go which currently happens when forced checkpointing writes out
  and waits upon 3/4 of the journal's size worth of data.