InnoDB hang in buf_flush_wait_flushed(), on log checkpoint

On our CI systems, on builders that run on real storage and not RAM disk, we see occasional failures of the IMPORT TABLESPACE tests, because a wait for a log checkpoint is hanging with a stack trace like this:

buf_flush_wait_flushed

log_make_checkpoint

row_import_cleanup

row_import_for_mysql

Actually, the checkpoint there should be unnecessary, but that is not the main point here.
I was able to reproduce this on ext4fs on storage with 4096-byte physical block size using a RelWithDebInfo build. Previous attempts on an NVMe with 512-byte physical block size failed.

innodb.innodb-wl5522 'innodb,strict_crc32' w11 [ 7 fail ]  timeout after 900 seconds

The test invocation was:

./mtr --parallel=100 --repeat=100 {innodb.innodb-wl5522{,-1},innodb_zip.wl5522_zip}{,,,,,,,,,,,,,,,,,,}

I think that applying the first commit of MDEV-26827 (to invoke buf_flush_list() from fewer threads) might fix this. That commit is also removing the log_make_checkpoint() call from row_import_cleanup(), but when testing the fix we obviously must retain that call, because our test case cannot fail if that call is not present.