UPSTREAM: kasan: fix tag for large allocations when using CONFIG_SLAB

If an object is allocated on a tail page of a multi-page slab, kasan
will get the wrong tag because page->s_mem is NULL for tail pages.  I'm
not quite sure what the user-visible effect of this might be.

Bug: 254441685
Link: https://lkml.kernel.org/r/20211001024105.3217339-1-willy@infradead.org


Fixes: 7f94ffbc4c6a ("kasan: add hooks implementation for tag-based mode")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Marco Elver <elver@google.com>
Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit 820a1e6e87ccaa6c0c77ac7d79d05beec3f8cb88)
Signed-off-by: Lee Jones <joneslee@google.com>
Change-Id: I1e9c12a04670ad52498e09ae85430e497f3d7816