Limit the number of the pages that are sampled so it is never greater
than the total number of pages in the index.

The parameter that specifies the number of pages to test is global for
all tables. By limiting it this way we allow the user to set it "high"
to suit "large" tables and to avoid unnecessary work for "small" tables
(e.g. doing 100 dives in a table that has 5 pages, obviously testing
some pages more than once).

Suggested by:	Ken
Approved by:	Marko