drm/nouveau: check pushbuffer bounds in ioctl

Currently there is no check that the pushbuffer request bounds are inside
the TTM BO.

This allows to instruct the kernel to do relocations on user-selected
addresses, since the relocation bounds checking relies on the request
bounds.

This can oops the kernel accidentally and is easily exploitable.

This patch adds bound checking and alignment checking for ->offset and
->nr_dwords.

It also makes some variables unsigned, which should have no effect,
but prevents possible bounds checking problems.

Signed-off-by: Luca Barbieri <luca@luca-barbieri.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>