libbpf: fix errno is overwritten after being closed.

In the ensure_good_fd function, if the fcntl function succeeds but
the close function fails, ensure_good_fd returns a normal fd and
sets errno, which may cause users to misunderstand. The close
failure is not a serious problem, and the correct FD has been
handed over to the upper-layer application. Let's restore errno here.
Signed-off-by: Xin Liu <liuxin350@huawei.com>
Link: https://lore.kernel.org/r/20221223133618.10323-1-liuxin350@huawei.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/lib/bpf/libbpf_internal.h

@@ -543,10 +543,9 @@ static inline int ensure_good_fd(int fd)
 		fd = fcntl(fd, F_DUPFD_CLOEXEC, 3);
 		saved_errno = errno;
 		close(old_fd);
-		if (fd < 0) {
+		errno = saved_errno;
+		if (fd < 0)
 			pr_warn("failed to dup FD %d to FD > 2: %d\n", old_fd, -saved_errno);
-			errno = saved_errno;
-		}
 	}
 	return fd;
 }