mem/pidfd: fix poll retry error checking

One should never rely on errno if libc syscall is successful. We can
either see an errno set from some previous failed syscall or even errno
set by a this successful libc syscall. So lets check ret first.

Fixes: 1ccdaf47 ("criu: add pidfd based pid reuse detection for RPC
clients")
Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>

criu/mem.c

@@ -246,7 +246,7 @@ static int check_pidfd_entry_state(struct pidfd_entry *entry)
 
 	while (1) {
 		ret = poll(&pollfd, 1, 0);
-		if (errno == EINTR && restart_cnt < MAX_RESTARTS) {
+		if (ret == -1 && errno == EINTR && restart_cnt < MAX_RESTARTS) {
 			restart_cnt++;
 			continue; /* restart polling */
 		}