Search for SLRU page only in its own bank

One of the two slot scans in SlruSelectLRUPage was not walking only the
slots in the specific bank where the buffer could be; change it to do
that.

Oversight in 53c2a97a92.

Author: Sergey Sargsyan <sergey.sargsyan.2001@gmail.com>
Discussion: https://postgr.es/m/18582-5f301dd30ba91a38@postgresql.org

src/backend/access/transam/slru.c

@@ -1184,7 +1184,7 @@ SlruSelectLRUPage(SlruCtl ctl, int64 pageno)
 		Assert(LWLockHeldByMe(SimpleLruGetBankLock(ctl, pageno)));
 
 		/* See if page already has a buffer assigned */
-		for (int slotno = 0; slotno < shared->num_slots; slotno++)
+		for (int slotno = bankstart; slotno < bankend; slotno++)
 		{
 			if (shared->page_status[slotno] != SLRU_PAGE_EMPTY &&
 				shared->page_number[slotno] == pageno)