drm/etnaviv: limit submit sizes

Currently we allow rediculous amounts of kernel memory being allocated
via the etnaviv GEM_SUBMIT ioctl, which is a pretty easy DoS vector. Put
some reasonable limits in to fix this.

The commandstream size is limited to 64KB, which was already a soft limit
on older kernels after which the kernel only took submits on a best effort
base, so there is no userspace that tries to submit commandstreams larger
than this. Even if the whole commandstream is a single incrementing address
load, the size limit also limits the number of potential relocs and
referenced buffers to slightly under 64K, so use the same limit for those
arguments. The performance monitoring infrastructure currently supports
less than 50 performance counter signals, so limiting them to 128 on a
single submit seems like a reasonably future-proof number for now. This
number can be bumped if needed without breaking the interface.

Cc: stable@vger.kernel.org
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>