drm/i915/gem: Hook user-extensions upto MMAP_OFFSET_IOCTL

Call i915_user_extensions() to validate the arg->extensions pointer, and
so return consistent error numbers for the future.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191204162803.3841140-1-chris@chris-wilson.co.uk