safe_mutex: Found wrong usage of mutex 'LOCK_thd_data' and 'LOCK_plugin'

Issue initially observed during testing of MDEV-28856, but present in trunk also:

safe_mutex: Found wrong usage of mutex 'LOCK_thd_data' and 'LOCK_plugin'

Mutex currently locked (in reverse order):

LOCK_plugin                       /test/git-bisect/preview-11.3-preview_MDEV-28856_patched_dbg/sql/sql_plugin.cc  line 2481

LOCK_thd_data                     /test/git-bisect/preview-11.3-preview_MDEV-28856_patched_dbg/sql/sql_class.h  line 3967

LOCK_thd_kill                     /test/git-bisect/preview-11.3-preview_MDEV-28856_patched_dbg/sql/sql_class.h  line 3966

The issue can be seen either during normal operation, or during SHUTDOWN. Possibly these are different issues leading to the same outcome.
The issues is highly sporadic, yet regularly seen. Testcase reduction has proven to be very difficult. However, a single occurrence reduced to this SHUTDOWN-based testcase:

INSTALL PLUGIN spider SONAME 'ha_spider.so';

CREATE TABLE t (c INT) ENGINE=Spider;

SET max_session_mem_used=8192;

UNINSTALL SONAME 'ha_spider';

SHUTDOWN;

This testcase may not be representative of all issues and should not be taken as a typical example, as this issue is during SHUTDOWN and reduced readily compared with the other sporadic occurrences, which do not reduce. Still, it may provide an indication in the code as to the actual bug(s).