Fix Bug#40760 "set global innodb_thread_concurrency = 0;" is not safe

The config param innodb_thread_concurrency is dynamically set and is
read when a thread enters/exits innodb. If the value is changed between
the enter and exit time the behaviour becomes erratic.
The fix is not to use srv_thread_concurrency when exiting, instead use
the flag trx->declared_to_be_inside_innodb.

rb://57

Approved by: Marko