Adjust some incorrect *GetDatum() macros

Adjust some incorrect *GetDatum() macros

This reverts portions of commit 6dcfac9696cb, which is wrong in trying
to use a *GetDatum() that matches with the C types of the values read.
*GetDatum() should match with the output argument types of the SQL
functions.

The portions of 6dcfac9696cb that are right regarding this rule are:
- gistget.c, where the GiST support functions use DatumGetUInt16() to
retrieve the strategy number.
- The BRIN code for strategynum, used in syscache lookups.

The adjustments done in this commit are for pageinspect, pg_buffercache
and pg_lock_status().

While double-checking the whole state of the tree regarding non-matching
pairs of DatumGet*() and *GetDatum(), I have found much more code paths
that are incorrect, unrelated to 6dcfac9696cb.  These may be adjusted in
the future, in a different patch (perhaps not for v19, as we are already
past feature freeze).

Reported-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/97f9375a-be61-4272-a44d-408337fe8fa6@eisentraut.org
Discussion: https://postgr.es/m/CAJ7c6TMcGu8qmRe1gZfJ-gOzVnZq-t=fwn-UuyStx1w6ZyydMw@mail.gmail.com