linux-user: Check for bad event numbers in epoll_wait

The kernel checks that the maxevents parameter to epoll_wait
is non-negative and not larger than EP_MAX_EVENTS. Add this
check to our implementation, so that:
 * we fail these cases EINVAL rather than EFAULT
 * we don't pass negative or overflowing values to the
   lock_user() size calculation

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

linux-user/syscall_defs.h

@@ -2585,6 +2585,9 @@ struct target_epoll_event {
     abi_uint events;
     target_epoll_data_t data;
 } TARGET_EPOLL_PACKED;
+
+#define TARGET_EP_MAX_EVENTS (INT_MAX / sizeof(struct target_epoll_event))
+
 #endif
 struct target_rlimit64 {
     uint64_t rlim_cur;