drm/xe/userptr: Fix an incorrect assert

The assert incorrectly checks the total length processed which
can in fact be greater than the number of pages. Fix.

Fixes: ea3e66d2 ("drm/xe/hmm: Don't dereference struct page pointers without notifier lock")
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250307100109.21397-1-thomas.hellstrom@linux.intel.com