random: vDSO: reject unknown getrandom() flags

Like the getrandom() syscall, vDSO getrandom() must also reject unknown
flags. [1]

It would be possible to return -EINVAL from vDSO itself, but in the
possible case that a new flag is added to getrandom() syscall in the
future, it would be easier to get the behavior from the syscall, instead
of erroring until the vDSO is extended to support the new flag or
explicitly falling back.

[1] Designing the API: Planning for Extension
    https://docs.kernel.org/process/adding-syscalls.html#designing-the-api-planning-for-extension



Signed-off-by: Yann Droneaud <yann@droneaud.fr>
[Jason: reworded commit message]
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>