aufs: update kmalloc_index to v5.14+ restrictions

commit 588c7fa0 [mm, slub: change run-time assertion in
kmalloc_index() to compile-time], means that the non-constant
aufs kmalloc call, should go (temporarily) to the __kmalloc_index
directly, so the compile time BUG is not triggered.

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>