seccomp: Refactor notification handler to prepare for new semantics

This refactors the user notification code to have a do / while loop around
the completion condition. This has a small change in semantic, in that
previously we ignored addfd calls upon wakeup if the notification had been
responded to, but instead with the new change we check for an outstanding
addfd calls prior to returning to userspace.

Rodrigo Campos also identified a bug that can result in addfd causing
an early return, when the supervisor didn't actually handle the
syscall [1].

[1]: https://lore.kernel.org/lkml/[email protected]/

Fixes: 7cf97b125455 ("seccomp: Introduce addfd ioctl to seccomp user notifier")
Signed-off-by: Sargun Dhillon <[email protected]>
Acked-by: Tycho Andersen <[email protected]>
Acked-by: Christian Brauner <[email protected]>
Signed-off-by: Kees Cook <[email protected]>
Tested-by: Rodrigo Campos <[email protected]>
Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]

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