In commit e71e6133 (MDEV-24671),
lock_sys.wait_mutex was moved above lock_sys.mutex
(which was later replaced with lock_sys.latch) in the latching order.

In commit 7cf4419f (MDEV-24789),
a potential hang was introduced to Galera. The function lock_wait()
would hold lock_sys.wait_mutex while invoking wsrep_is_BF_lock_timeout(),
which in turn could acquire LockMutexGuard for some diagnostic printout.

wsrep_is_BF_lock_timeout(): Do not invoke trx_print_latched() or
LockMutexGuard.

lock_sys_t::wr_lock(), lock_sys_t::rd_lock(): Assert that the current
thread is not holding lock_sys.wait_mutex.
Unfortunately, RW-locks are not covered by SAFE_MUTEX.

Reviewed by: Jan Lindström