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Both wait_for_commit() and wait_for_writer() are checking the
condition out of the mutex lock.

This refactors code a bit to be lock safe.
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>