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

@@ -152,6 +152,8 @@ 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);
@@ -265,6 +267,8 @@ pg_buffercache_summary(PG_FUNCTION_ARGS)
 		BufferDesc *bufHdr;
 		uint32		buf_state;
 
+		CHECK_FOR_INTERRUPTS();
+
 		/*
 		 * This function summarizes the state of all headers. Locking the
 		 * buffer headers wouldn't provide an improved result as the state of
@@ -325,6 +329,8 @@ pg_buffercache_usage_counts(PG_FUNCTION_ARGS)
 		uint32		buf_state = pg_atomic_read_u32(&bufHdr->state);
 		int			usage_count;
 
+		CHECK_FOR_INTERRUPTS();
+
 		usage_count = BUF_STATE_GET_USAGECOUNT(buf_state);
 		usage_counts[usage_count]++;