Problem was that the handler call ::extra(HA_EXTRA_CACHE) was cached
but the ::extra(HA_EXTRA_PREPARE_FOR_UPDATE) was not.

Solution was to also cache the other call and forward it when moving
to a new partition to scan.

mysql-test/r/partition.result:
  test result
mysql-test/t/partition.test:
  New test from bug report.
sql/ha_partition.cc:
  cache the HA_EXTRA_PREPARE_FOR_UPDATE just like HA_EXTRA_CACHE.
sql/ha_partition.h:
  Added cache flag for HA_EXTRA_PREPARE_FOR_UPDATE