The test innodb.instant_alter_crash is exercising crash recovery
for instant ALTER operations. Unfortunately, for some reason the
redo log was not being flushed as expected (to be analyzed in MDEV-18009)
and this error was not detected earlier.

btr_cur_trim_alter_metadata(): New function for the special case of
rolling back an ALTER TABLE operation such that the table will remain
in the MDEV-15562 format (not rolling back to the MDEV-11369 format).
In this case, we must restore the old number of columns from the
old metadata BLOB page.