ha_innobase::extra() invokes check_trx_exists() unconditionally even for
not supported operations. check_trx_exists() creates and registers trx_t
object if THD does not contain pointer to it. If ha_innobase::extra() does
not support some operation, it just invokes check_trx_exists() and quites.
If check_trx_exists() creates and registers new trx_t object for such
operation, it will never be freed and deregistered.

For example, if ha_innobase::extra() is invoked from purge thread with
operation = HA_EXTRA_IS_ATTACHED_CHILDREN, like it goes in
gcol.gcol_purge test, trx_t object will be registered, but not
deregisreted, and this causes innodb.gap_lock failure, as "SHOW ENGINE
INNODB STATUS" shows information about unexpected transaction at the end
of trx_sys.trx_list.

The fix is not to invoke check_trx_exists() for unsupported operations
in ha_innobase::extra().

Reviewed by: Marko Mäkelä