libbpf: Restore errno after pr_warn.

pr_warn calls into user-provided callback, which can clobber errno, so
`errno = saved_errno` should happen after pr_warn.

Fixes: 07453245620c ("libbpf: fix errno is overwritten after being closed.")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

src/libbpf_internal.h

@@ -544,8 +544,10 @@ static inline int ensure_good_fd(int fd)
 		saved_errno = errno;
 		close(old_fd);
 		errno = saved_errno;
-		if (fd < 0)
+		if (fd < 0) {
 			pr_warn("failed to dup FD %d to FD > 2: %d\n", old_fd, -saved_errno);
+			errno = saved_errno;
+		}
 	}
 	return fd;
 }