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

start_this_handle() takes into account t_outstanding_credits when calculating
log free space, but journal_next_log_block() accounts for blocks being logged
also.  Hence, blocks are accounting twice.  This effectively reduces the
amount of log space available to transactions and forces more commits.

Fix it by decrementing t_outstanding_credits each time we allocate a new
journal block.