free space % calculation in pgstathashindex

Hi,

While working on - [1] (one of the bug reported for hash index), i
noticed that the percentage of free space shown in 'free_percent'
column of pgstathashindex() is incorrect for some of the boundary
cases. This is how the free space percentage is calculated by
pgstathashindex(),

1) /* Count unused pages as free space. */
stats.free_space += stats.unused_pages * stats.space_per_page;

2) /* Total space available for tuples excludes the metapage and the
bitmap pages */
total_space = (nblocks - (stats.bitmap_pages + 1)) * stats.space_per_page;

3) Calculate the percentage of free space in hash index table.
free_percent = 100.0 * stats.free_space / total_space;

In step #1, assuming '*' as an arithmetic operator, the left operand
i.e. 'stats.unused_pages' is of type uint32 whereas the right operand
i.e. 'stats.space_per_page' is of type int32 and arithmetic
conversions of the ANSI C standard states: 'if either operand has type
unsigned int, the other operand is converted to unsigned int' which
means stats.space_per_page would be converted to uint32 and then the
multiplication would be performed. But, if the result of
multiplication is greater than the max value accepted by 32 bits
number, we would get an incorrect result. Hence, it is important to do
proper typecasting of operands before performing any arithmetic
operation. In this case, i would typecast both stats.unused_pages and
stats.space_per_page to uint64 and the
perform multiplication on them. Similar handling needs to be done in
step #2 as well.

Attached is the patch that fixes this.

[1] - https://www.postgresql.org/message-id/20170704105728.mwb72jebfmok2nm2%40zip.com.au
[2] - http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.faqs/ka9483.html