buf_page_get_gen().  This saves one mutex operation per block request.

buf_page_get_gen(), various macros and functions: Add parameter zip_size.

btr_node_ptr_get_child(): Add parameter index.

fil_space_get_latch(): Add optional output parameter zip_size.

fil_space_get_zip_size(): Return 0 for space id==0, because the
system tablespace is never compressed.

fsp_header_init(): Remove the parameter zip_size.

ibuf_free_excess_pages(): Remove the parameter zip_size.

trx_rseg_t, trx_undo_t: Add field zip_size.

xdes_lst_get_next(): Remove, unused.