btrfs: move variables for clustered allocation into find_free_extent_ctl

Move "last_ptr" and "use_cluster" into struct find_free_extent_ctl, so
that hook functions for clustered allocator can use these variables.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
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>