WL#6326 in MariaDB 10.2.2 introduced a potential hang on purge or rollback
when an index tree is being shrunk by multiple levels.

This fix is based on
mysql/mysql-server@f2c58526300c0d84837effa26d37cbd5d2694967
with the main difference that our version of the test case uses
DEBUG_SYNC instrumentation on ROLLBACK, not on purge.

btr_cur_will_modify_tree(): Simplify the check further.
This is the actual bug fix.

row_undo_mod_remove_clust_low(), row_undo_mod_clust(): Add DEBUG_SYNC
instrumentation for the test case.