MDEV-40080: innodb_log_archive=ON corruption

[dr-m]

log_t::write_checkpoint(): Extend the the correct file.

log_t::archived_mmap_switch_prepare(): Tolerate a near-concurrent buf_flush_archive_create() from the buf_flush_page_cleaner() thread.

This fixes up #4817. I tested both changes with the test case in MDEV-40080. The first one is for the WITH_INNODB_PMEM=OFF code path (pwrite based log writes), and the second one is for the memory-mapped log writes.

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