btrfs: move struct io_ctl to free-space-cache.h

The io_ctl structure is used for free space management, and used only by
the v1 space cache code, but unfortunatlly the full definition is
required by block-group.h so it can't be moved to free-space-cache.c
without additional changes.

Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: David Sterba <dsterba@suse.com>