linux-user: Use qemu_set_cloexec() to mark pidfd as FD_CLOEXEC

linux-user: Use qemu_set_cloexec() to mark pidfd as FD_CLOEXEC

In the linux-user do_fork() function we try to set the FD_CLOEXEC
flag on a pidfd like this:

    fcntl(pid_fd, F_SETFD, fcntl(pid_fd, F_GETFL) | FD_CLOEXEC);

This has two problems:
 (1) it doesn't check errors, which Coverity complains about
 (2) we use F_GETFL when we mean F_GETFD

Deal with both of these problems by using qemu_set_cloexec() instead.
That function will assert() if the fcntls fail, which is fine (we are
inside fork_start()/fork_end() so we know nothing can mess around
with our file descriptors here, and we just got this one from
pidfd_open()).

(As we are touching the if() statement here, we correct the
indentation.)

Coverity: CID 1508111
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20250711141217.1429412-1-peter.maydell@linaro.org>