xen: fix page_list_splice()

xen: fix page_list_splice()

Other than in __list_splice(), the first element's prev pointer
doesn't need adjustment here - it already is PAGE_LIST_NULL. Rather
than fixing the assignment (to formally match __list_splice()), simply
assert that this assignment is really unnecessary.

Reported-by: Jisoo Yang <jisooy@gmail.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Also assert that the prev pointers are both PAGE_LIST_NULL.

Signed-off-by: Keir Fraser <keir@xen.org>
Committed-by: Keir Fraser <keir@xen.org>