Fix BUG#19542 "InnoDB doesn't increase the Handler_read_prev counter".


innobase/os/os0file.c:
  Applied innodb-5.0-ss547 snapshot.
   Check the page trailers also after writing to disk.
   This improves the chances of diagnosing Bug 18886.
   os_file_check_page_trailers(): New function for checking
   that two copies of the LSN stamped on the pages match.
   os_aio_simulated_handle(): Call os_file_check_page_trailers()
   before and after os_file_write().
sql/ha_innodb.cc:
  Applied innodb-5.0-ss547 snapshot.
   Increment statistic counter in ha_innobase::index_prev().