The function log_free_check() is not supposed to be invoked while
the caller is holding any InnoDB synchronization objects, such as
buffer page latches, tablespace latches, index tree latches, or
in this case, rseg->mutex (rseg->latch in 10.6 or later).

A hang was reported in 10.6 where several threads were waiting for
an rseg->latch that had been exclusively acquired in
trx_purge_truncate_history(), which invoked log_free_check() inside
trx_purge_truncate_rseg_history(). Because the threads that were
waiting for the rseg->latch were holding exclusive latches on some
index pages, log_free_check() was unable to advance the checkpoint
because those index pages could not be written out.

trx_purge_truncate_history(): Invoke log_free_check() before
acquiring the rseg->mutex and invoking trx_purge_free_segment().

trx_purge_free_segment(): Do not invoke log_free_check() in order
to avoid a deadlock.