drm/vmwgfx: Fix memory leak in vmw_mksstat_add_ioctl()

If the copy of the description string from userspace fails, then the page
for the instance descriptor doesn't get freed before returning -EFAULT,
which leads to a memleak.

Fixes: 7a7a933e ("drm/vmwgfx: Introduce VMware mks-guest-stats")
Signed-off-by: Rafael Mendonca <rafaelmendsr@gmail.com>
Reviewed-by: Martin Krastev <krastevm@vmware.com>
Signed-off-by: Zack Rusin <zackr@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220916204751.720716-1-rafaelmendsr@gmail.com