linux-user: Range check the nfds argument to ppoll syscall

Do an initial range check on the ppoll syscall's nfds argument,
to avoid possible overflow in the calculation of the lock_user()
size argument. The host kernel will later apply the rather lower
limit based on RLIMIT_NOFILE as appropriate.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
