Add CHECK_FOR_INTERRUPTS in Evict{Rel,All}UnpinnedBuffers.

This commit adds CHECK_FOR_INTERRUPTS to the shared buffer iteration
loops in EvictRelUnpinnedBuffers and EvictAllUnpinnedBuffers. These
functions, used by pg_buffercache's pg_buffercache_evict_relation and
pg_buffercache_evict_all, can now be interrupted during long-running
operations.

Backpatch to version 18, where these functions and their corresponding
pg_buffercache functions were introduced.

Author: Yuhang Qiu <iamqyh@gmail.com>
Discussion: https://postgr.es/m/8DC280D4-94A2-4E7B-BAB9-C345891D0B78%40gmail.com
Backpatch-through: 18

src/backend/storage/buffer/bufmgr.c

@@ -6685,6 +6685,8 @@ EvictAllUnpinnedBuffers(int32 *buffers_evicted, int32 *buffers_flushed,
 		uint32		buf_state;
 		bool		buffer_flushed;
 
+		CHECK_FOR_INTERRUPTS();
+
 		buf_state = pg_atomic_read_u32(&desc->state);
 		if (!(buf_state & BM_VALID))
 			continue;
@@ -6735,6 +6737,8 @@ EvictRelUnpinnedBuffers(Relation rel, int32 *buffers_evicted,
 		uint32		buf_state = pg_atomic_read_u32(&(desc->state));
 		bool		buffer_flushed;
 
+		CHECK_FOR_INTERRUPTS();
+
 		/* An unlocked precheck should be safe and saves some cycles. */
 		if ((buf_state & BM_VALID) == 0 ||
 			!BufTagMatchesRelFileLocator(&desc->tag, &rel->rd_locator))