mm, slub: avoid zeroing kmalloc redzone

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

mm, slub: avoid zeroing kmalloc redzone

Since commit 946fa0dbf2d8 ("mm/slub: extend redzone check to extra
allocated kmalloc space than requested"), setting orig_size treats
the wasted space (object_size - orig_size) as a redzone. However with
init_on_free=1 we clear the full object->size, including the redzone.

Additionally we clear the object metadata, including the stored orig_size,
making it zero, which makes check_object() treat the whole object as a
redzone.

These issues lead to the following BUG report with "slub_debug=FUZ
init_on_free=1":

[ 0.000000] =============================================================================
[ 0.000000] BUG kmalloc-8 (Not tainted): kmalloc Redzone overwritten
[ 0.000000] -----------------------------------------------------------------------------
[ 0.000000]
[ 0.000000] 0xffff000010032858-0xffff00001003285f @offset=2136. First byte 0x0 instead of 0xcc
[ 0.000000] FIX kmalloc-8: Restoring kmalloc Redzone 0xffff000010032858-0xffff00001003285f=0xcc
[ 0.000000] Slab 0xfffffdffc0400c80 objects=36 used=23 fp=0xffff000010032a18 flags=0x3fffe0000000200(workingset|node=0|zone=0|lastcpupid=0x1ffff)
[ 0.000000] Object 0xffff000010032858 @offset=2136 fp=0xffff0000100328c8
[ 0.000000]
[ 0.000000] Redzone ffff000010032850: cc cc cc cc cc cc cc cc ........
[ 0.000000] Object ffff000010032858: cc cc cc cc cc cc cc cc ........
[ 0.000000] Redzone ffff000010032860: cc cc cc cc cc cc cc cc ........
[ 0.000000] Padding ffff0000100328b4: 00 00 00 00 00 00 00 00 00 00 00 00 ............
[ 0.000000] CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Not tainted 6.11.0-rc3-next-20240814-00004-g61844c55c3f4 #144
[ 0.000000] Hardware name: NXP i.MX95 19X19 board (DT)
[ 0.000000] Call trace:
[ 0.000000] dump_backtrace+0x90/0xe8
[ 0.000000] show_stack+0x18/0x24
[ 0.000000] dump_stack_lvl+0x74/0x8c
[ 0.000000] dump_stack+0x18/0x24
[ 0.000000] print_trailer+0x150/0x218
[ 0.000000] check_object+0xe4/0x454
[ 0.000000] free_to_partial_list+0x2f8/0x5ec

To address the issue, use orig_size to clear the used area. And restore
the value of orig_size after clear the remaining area.

When CONFIG_SLUB_DEBUG not defined, (get_orig_size()' directly returns
s->object_size. So when using memset to init the area, the size can simply
be orig_size, as orig_size returns object_size when CONFIG_SLUB_DEBUG not
enabled. And orig_size can never be bigger than object_size.