Bulk REPLACE or bulk INSERT ... ON DUPLICATE KEY UPDATE may
break dynamic record MyISAM table.

The problem is limited to bulk REPLACE and INSERT ... ON
DUPLICATE KEY UPDATE, because only these operations may
be done via UPDATE internally and may request write cache.

When flushing write cache, MyISAM may write remaining
cached data at wrong position. Fixed by requesting write
cache to seek to a correct position.

mysql-test/r/myisam.result:
  A test case for BUG#49628.
mysql-test/t/myisam.test:
  A test case for BUG#49628.
storage/myisam/mi_dynrec.c:
  delete_dynamic_record() may change data file position.
  IO cache must be notified as it may still have cached
  data, which has to be flushed later.