btrfs: simplify tracking progress for the extent map shrinker

Now that the extent map shrinker can only be run by a single task (as a
work queue item) there is no need to keep the progress of the shrinker
protected by a spinlock and passing the progress to trace events as
parameters. So remove the lock and simplify the arguments for the trace
events.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>