drm/xe/vm: don't ignore error when in_kthread

If GUP fails and we are in_kthread, we can have pinned = 0 and ret = 0.
If that happens we call sg_alloc_append_table_from_pages() with n_pages
= 0, which is not well behaved and can trigger:

kernel BUG at include/linux/scatterlist.h:115!

depending on if the pages array happens to be zeroed or not. Even if we
don't hit that it crashes later when trying to dma_map the returned
table.

Fixes: dd08ebf6

 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240202171435.427630-2-matthew.auld@intel.com