Change fastgetattr and heap_getattr to inline functions

Change fastgetattr and heap_getattr to inline functions

They were macros previously, but recent callsite additions made Coverity
complain about one of the assertions being always true.  This change
could have been made a long time ago, but the Coverity complain broke
the inertia.

Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Japin Li <japinli@hotmail.com>
Reviewed-by: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
Discussion: https://postgr.es/m/202203241021.uts52sczx3al@alvherre.pgsql