The function innodb_show_mutex_status() is the only ultimate caller of
LatchCounter::iterate() via MutexMonitor::iterate(). Because the call
is not protected by LatchCounter::m_mutex, any mutex_create() or
mutex_free() that is invoked concurrently during the execution, bad
things such as a crash could happen.

The most likely way for this to happen is buffer pool resizing,
which could cause buf_block_t::mutex (which existed before MDEV-15053)
to be created or freed. We could also register InnoDB mutexes in
TrxFactory::init() if trx_pools needs to grow.

The view INFORMATION_SCHEMA.INNODB_MUTEXES is not affected, because it
only displays information about rw-locks, not mutexes.

This commit intentionally touches also MutexMonitor::iterate()
and the only code that interfaces with LatchCounter::iterate()
to make it clearer for future readers that the scattered code
that is obfuscated by templates belongs together.

This is based on
mysql/mysql-server@273a93396f49c7e0a8b07b260128d9a990c2b154