log_buf_pool_get_oldest_modification(): Acquire
log_sys_t::flush_order_mutex in order to prevent a race condition
that was introduced in
commit 1a6f708e (MDEV-15058).

Before that change, log_buf_pool_get_oldest_modification()
was protected by both log_sys.mutex and log_sys.flush_order_mutex
like it was supposed to be ever since
commit a52c4820 (MySQL 5.5.10).

buf_pool_t::get_oldest_modification(): Replaces
buf_pool_get_oldest_modification(), to emphasize that
log_sys.flush_order_mutex must be acquired by the caller if needed.

log_close(): Invoke log_buf_pool_get_oldest_modification()
in order to ensure a clean shutdown.

The scenario of the race condition is as follows:

1. The buffer pool is clean (no writes are pending).
2. mtr_add_dirtied_pages_to_flush_list() releases log_sys.mutex.
3. log_buf_pool_get_oldest_modification() observes that the
buffer pool is clean and returns log_sys.lsn.
4. log_checkpoint() completes, writing a wrong checkpoint header
according to which everything up to log_sys.lsn was clean.
5. mtr_add_dirtied_pages_to_flush_list() completes the execution
of mtr_memo_note_modifications(), releases the page latches and
the flush_order_mutex.
6. On a subsequent log_checkpoint(), the assertion could fail
if the page modifications had not been flushed yet.

The failing assertion (which is valid) was added in MySQL 5.7
mysql/mysql-server@5c6c6ec69336369487dfc080a6980089b4e1a3c2
and merged to MariaDB Server 10.2.2 in
commit fec844ac.