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 4835458 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/

Author: feodor

src/backend/storage/buffer/bufmgr.c

@@ -1595,9 +1595,21 @@ PinBuffer(BufferDesc *buf, BufferAccessStrategy strategy)
 			/* increase refcount */
 			buf_state += BUF_REFCOUNT_ONE;
 
-			/* increase usagecount unless already max */
-			if (BUF_STATE_GET_USAGECOUNT(buf_state) != BM_MAX_USAGE_COUNT)
-				buf_state += BUF_USAGECOUNT_ONE;
+			if (strategy == NULL)
+			{
+				/* Default case: increase usagecount unless already max. */
+				if (BUF_STATE_GET_USAGECOUNT(buf_state) < BM_MAX_USAGE_COUNT)
+					buf_state += BUF_USAGECOUNT_ONE;
+			}
+			else
+			{
+				/*
+				 * Ring buffers shouldn't evict others from pool.  Thus we
+				 * don't make usagecount more than 1.
+				 */
+				if (BUF_STATE_GET_USAGECOUNT(buf_state) == 0)
+					buf_state += BUF_USAGECOUNT_ONE;
+			}
 
 			if (pg_atomic_compare_exchange_u32(&buf->state, &old_buf_state,
 											   buf_state))