MDEV-22110 preparation: Remove some unused function parameters

xdes_get_state(), fseg_get_nth_frag_page_no(),
fseg_find_free_frag_page_slot(), fseg_find_last_used_frag_page_slot(),
fseg_get_n_frag_pages(), fseg_n_reserved_pages_low(),
fseg_print_low(): Remove the unused parameter mtr, and add
a const qualifier to the pointer to the buffer pool page frame.