btrfs: drop unnecessary arguments from clustered allocation functions

Now that, find_free_extent_clustered() and find_free_extent_unclustered()
can access "last_ptr" from the "clustered" variable, we can drop it from
the arguments.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>