Revert unintentional change in increasing usage count during pin of buffers,

Revert unintentional change in increasing usage count during pin of buffers,
this makes buffer access strategy have no effect.
Change was a part of commit 48354581a49c30f5757c203415aa8412d85b0f70 during 9.6
release cycle, so backpath to 9.6

Reported-by: Jim Nasby
Author: Alexander Korotkov
Reviewed-by: Jim Nasby, Andres Freund

https://commitfest.postgresql.org/13/1029/