Mark buffers as defined to Valgrind consistently.

Make PinBuffer() mark buffers as defined to Valgrind unconditionally,
including when the buffer header spinlock must be acquired.  Failure to
handle that case could lead to false positive reports from Valgrind.

This theoretically creates a risk that we'll mark buffers defined even
when external callers don't end up with a buffer pin.  That seems
perfectly acceptable, though, since in general we make no guarantees
about buffers that are unsafe to access being reliably marked as unsafe.

Oversight in commit 1e0dfd16, which added valgrind buffer access
instrumentation.

src/backend/storage/buffer/bufmgr.c

@@ -1636,10 +1636,12 @@ PinBuffer(BufferDesc *buf, BufferAccessStrategy strategy)
 				result = (buf_state & BM_VALID) != 0;
 
 				/*
-				 * If we successfully acquired our first pin on this buffer
-				 * within this backend, mark buffer contents defined
+				 * Assume that we acquired a buffer pin for the purposes of
+				 * Valgrind buffer client checks (even in !result case) to
+				 * keep things simple.  Buffers that are unsafe to access are
+				 * not generally guaranteed to be marked undefined in any
+				 * case.
 				 */
-				if (result)
 				VALGRIND_MAKE_MEM_DEFINED(BufHdrGetBlock(buf), BLCKSZ);
 				break;
 			}