Fix incorrect close() call in dsm_impl_mmap().

One improbable error-exit path in this function used close() where
it should have used CloseTransientFile().  This is unlikely to be
hit in the field, and I think the consequences wouldn't be awful
(just an elog(LOG) bleat later).  But a bug is a bug, so back-patch
to 9.4 where this code came in.

Pan Bian

Discussion: https://postgr.es/m/152056616579.4966.583293218357089052@wrigleys.postgresql.org

src/backend/storage/ipc/dsm_impl.c

@@ -924,7 +924,7 @@ dsm_impl_mmap(dsm_op op, dsm_handle handle, Size request_size,
 
 		/* Back out what's already been done. */
 		save_errno = errno;
-		close(fd);
+		CloseTransientFile(fd);
 		if (op == DSM_OP_CREATE)
 			unlink(name);
 		errno = save_errno;