mm/slub: reset KASAN tag in defer_free() before accessing freed memory

In the Linux kernel, the following vulnerability has been resolved:

mm/slub: reset KASAN tag in defer_free() before accessing freed memory

When CONFIG_SLUB_TINY is enabled, kfree_nolock() calls kasan_slab_free()

before defer_free(). On ARM64 with MTE (Memory Tagging Extension),

kasan_slab_free() poisons the memory and changes the tag from the

original (e.g., 0xf3) to a poison tag (0xfe).

When defer_free() then tries to write to the freed object to build the

deferred free list via llist_add(), the pointer still has the old tag,

causing a tag mismatch and triggering a KASAN use-after-free report:

BUG: KASAN: slab-use-after-free in defer_free+0x3c/0xbc mm/slub.c:6537

Write at addr f3f000000854f020 by task kworker/u8:6/983

Pointer tag: [f3], memory tag: [fe]

Fix this by calling kasan_reset_tag() before accessing the freed memory.

This is safe because defer_free() is part of the allocator itself and is

expected to manipulate freed memory for bookkeeping purposes.