purge_sys_t::truncating_tablespace(): Clamp the
innodb_max_undo_log_size to the maximum number of pages
before converting the result into a 32-bit unsigned integer.

This fixes up commit f8c88d90 (MDEV-33213).

In later major versions, we would use 32-bit unsigned integer here
due to commit ca501ffb
and the code would crash also on 64-bit processors.

Reviewed by: Debarun Banerjee