In most cases, ha_partition forwards calls to extra() to all
locked_partitions. It doesn't make sense to forward some calls for
partitions that were pruned away.
This patch introduces ha_partition::loop_read_partitions and makes
these calls use it:

- ha_partition::extra_opt(HA_EXTRA_KEYREAD)
- ha_partition::extra(HA_EXTRA_KEYREAD)
- ha_partition::extra(HA_EXTRA_NO_KEYREAD)

Reviewed-by: Monty