Add CHECK_FOR_INTERRUPTS in contrib/pg_buffercache functions.

This commit adds CHECK_FOR_INTERRUPTS to loops iterating over shared
buffers in several pg_buffercache functions, allowing them to be
interrupted during long-running operations.

Backpatch to all supported versions. Add CHECK_FOR_INTERRUPTS to the
loop in pg_buffercache_pages() in all supported branches, and to
pg_buffercache_summary() and pg_buffercache_usage_counts() in version
16 and newer.

Author: SATYANARAYANA NARLAPURAM <satyanarlapuram@gmail.com>
Discussion: https://postgr.es/m/CAHg+QDcejeLx7WunFT3DX6XKh1KshvGKa8F5au8xVhqVvvQPRw@mail.gmail.com
Backpatch-through: 13

contrib/pg_buffercache/pg_buffercache_pages.c

@@ -11,6 +11,7 @@
 #include "access/htup_details.h"
 #include "catalog/pg_type.h"
 #include "funcapi.h"
+#include "miscadmin.h"
 #include "storage/buf_internals.h"
 #include "storage/bufmgr.h"
 
@@ -148,6 +149,8 @@ pg_buffercache_pages(PG_FUNCTION_ARGS)
 			BufferDesc *bufHdr;
 			uint32		buf_state;
 
+			CHECK_FOR_INTERRUPTS();
+
 			bufHdr = GetBufferDescriptor(i);
 			/* Lock each buffer header before inspecting. */
 			buf_state = LockBufHdr(bufHdr);