jbd2: avoid leaking transaction credits when unreserving handle

When reserved transaction handle is unused, we subtract its reserved
credits in __jbd2_journal_unreserve_handle() called from
jbd2_journal_stop(). However this function forgets to remove reserved
credits from transaction->t_outstanding_credits and thus the transaction
space that was reserved remains effectively leaked. The leaked
transaction space can be quite significant in some cases and leads to
unnecessarily small transactions and thus reducing throughput of the
journalling machinery. E.g. fsmark workload creating lots of 4k files
was observed to have about 20% lower throughput due to this when ext4 is
mounted with dioread_nolock mount option.

Subtract reserved credits from t_outstanding_credits as well.

CC: stable@vger.kernel.org
Fixes: 8f7d89f36829 ("jbd2: transaction reservation support")
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20200520133119.1383-3-jack@suse.cz
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

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