InnoDB is holding shared dict_sys.latch while waiting for FOREIGN KEY child table lock on DDL

In order to fix the race conditions MDEV-26217 and MDEV-26554, some code was added so that InnoDB could hold a shared dict_sys.latch while waiting for an exclusive lock on tables that are connected by FOREIGN KEY statements. This is not acceptable, because a lock wait can be blocked for a long time (worst case, indefinitely if innodb_lock_wait_timeout=100000000). If another thread tries to acquire an exclusive dict_sys.latch, it will block any other threads from acquiring a shared dict_sys.latch until the table lock wait has been resolved.

This bug can be fixed by changing lock_table_for_trx() so that whenever the caller is holding a shared dict_sys.latch, it will be released and reacquired around the call to lock_wait(). In this way, the lock object will be created or released while the table is protected by the shared dict_sys.latch. It is safe to temporarily release the dict_sys.latch, because tables on which lock objects exist cannot be evicted or dropped. In the callers, we have to take special care to ensure that dict_table_t::referenced_set is safe to traverse if dict_sys.latch was temporarily released.