[NTOSKRNL] Only allow SYSTEM to call NtSetUuidSeed()

[NTOSKRNL] Only allow SYSTEM to call NtSetUuidSeed()

Also, validate input buffer before attempting any operation on it.
This will avoid userland applications to be able to trigger an invalid
read in the kernel (and thus a BSOD).

Regarding access restriction, see:
https://stackoverflow.com/questions/1254244/need-access-to-ntsetuuidseed-from-a-non-localsystem-process

CORE-15460