Remove timerfd_create() syscall from the seccomp allow-list

[acatangiu]

With a bit of refactoring all timer fds could be created before booting up the guest. This would allow removing this syscall from the seccomp whitelist.

The timerfd_create syscall in conjunction with the timerfd_settime syscall could be abused to DoS attack the host kernel by creating many timers and continuously have them fire. Because the timerfd subsystem runs in the host kernel context and thus cannot be bounded by user CPU limits it can be used to starve other guests from CPU resources.

[acatangiu]

Fixed in #2606