seccomp: Support atomic "addfd + send reply"

Alban Crequy reported a race condition userspace faces when we want to
add some fds and make the syscall return them[1] using seccomp notify.

The problem is that currently two different ioctl() calls are needed by
the process handling the syscalls (agent) for another userspace process
(target): SECCOMP_IOCTL_NOTIF_ADDFD to allocate the fd and
SECCOMP_IOCTL_NOTIF_SEND to return that value. Therefore, it is possible
for the agent to do the first ioctl to add a file descriptor but the
target is interrupted (EINTR) before the agent does the second ioctl()
call.

This patch adds a flag to the ADDFD ioctl() so it adds the fd and
returns that value atomically to the target program, as suggested by
Kees Cook[2]. This is done by simply allowing
seccomp_do_user_notification() to add the fd and return it in this case.
Therefore, in this case the target wakes up from the wait in
seccomp_do_user_notification() either to interrupt the syscall or to add
the fd and return it....