When thread is BF aborted by high priority service, ULL (user level
locks need to be removed and released). Calling directly release of lock for
MDL_EXPLICIT type doesn't clear also `thd->ull_hash`. Method
`mysql_ull_cleanup` will properly clear all information about ULL locks
for thread.
Reviewed-by: Jan Lindström <jan.lindstrom@mariadb.com>