Fix checksum calculation in the new sorting GiST build.

Fix checksum calculation in the new sorting GiST build.

Since we're bypassing the buffer manager, we need to call
PageSetChecksumInplace() directly. As reported by Justin Pryzby.

In the passing, add RelationOpenSmgr() calls before all smgrwrite() and
smgrextend() calls. Tom added one before the first smgrextend() call in
commit c2bb287025, which seems to be enough, but let's play it safe and
do it before each one. That's how it's done in the similar code in
nbtsort.c, too.

Discussion: https://www.postgresql.org/message-id/20200920224446.GF30557@telsasoft.com