Be consistent and use unsigned long throughout the chunk copies to
avoid the inherent clumsiness of mixing integer types of different
widths and signs. Failing to take acount of a wider unsigned type when
using min_t can lead to treating it as a negative, only for it flip back
to a large unsigned value after passing a boundary check.

Fixes: ed13033f ("drm/i915/cmdparser: Only cache the dst vmap")
Testcase: igt/gen9_exec_parse/bb-large
Reported-by: "Candelaria, Jared" <jared.candelaria@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: "Candelaria, Jared" <jared.candelaria@intel.com>
Cc: "Bloomfield, Jon" <jon.bloomfield@intel.com>
Cc: <stable@vger.kernel.org> # v4.9+
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200928215942.31917-1-chris@chris-wilson.co.uk