Code review comment for lp:~laurynas-biveinis/percona-server/bp-split-5.6

Repushed. Changes from the 2nd push. Not a resubmission yet.

    - Removed ut_ad(mutex_own(&buf_pool->zip_free_mutex)) from
      buf_pool_contains_zip().
    - Fixed comment typos.
    - Reverted in_LRU_list and in_unzip_LRU_list changes, with the
      exception of an extra assert in buf_page_set_sticky().
    - Reverted spurious whitespace changes.
    - Removed spurious diagnostic printf from
      buf_pool_validate_instance().
    - Fixed locking in buf_block_try_discard_uncompressed().
    - Reverted redundant locking changes in buf_page_get_zip() and
      buf_page_get_gen().
    - Removed buf_enter_zip_mutex_for_page().
    - Removed os_atomic_load_ulint() uses from from
      buf_get_total_stat(), buf_print_instance(),
      buf_stat_get_pool_info(), buf_LRU_evict_from_unzip_LRU(), the
      last instance in buf_read_recv_pages(), and.
      srv_mon_process_existing_counter().