cachefiles: Fix KASAN slab-out-of-bounds in cachefiles_set_volume_xattr

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

cachefiles: Fix KASAN slab-out-of-bounds in cachefiles_set_volume_xattr

Use the actual length of volume coherency data when setting the
xattr to avoid the following KASAN report.

BUG: KASAN: slab-out-of-bounds in cachefiles_set_volume_xattr+0xa0/0x350 [cachefiles]
Write of size 4 at addr ffff888101e02af4 by task kworker/6:0/1347

CPU: 6 PID: 1347 Comm: kworker/6:0 Kdump: loaded Not tainted 5.18.0-rc1-nfs-fscache-netfs+ #13
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.14.0-4.fc34 04/01/2014
Workqueue: events fscache_create_volume_work [fscache]
Call Trace:
<TASK>
dump_stack_lvl+0x45/0x5a
print_report.cold+0x5e/0x5db
? __lock_text_start+0x8/0x8
? cachefiles_set_volume_xattr+0xa0/0x350 [cachefiles]
kasan_report+0xab/0x120
? cachefiles_set_volume_xattr+0xa0/0x350 [cachefiles]
kasan_check_range+0xf5/0x1d0
memcpy+0x39/0x60
cachefiles_set_volume_xattr+0xa0/0x350 [cachefiles]
cachefiles_acquire_volume+0x2be/0x500 [cachefiles]
? __cachefiles_free_volume+0x90/0x90 [cachefiles]
fscache_create_volume_work+0x68/0x160 [fscache]
process_one_work+0x3b7/0x6a0
worker_thread+0x2c4/0x650
? process_one_work+0x6a0/0x6a0
kthread+0x16c/0x1a0
? kthread_complete_and_exit+0x20/0x20
ret_from_fork+0x22/0x30
</TASK>

Allocated by task 1347:
kasan_save_stack+0x1e/0x40
__kasan_kmalloc+0x81/0xa0
cachefiles_set_volume_xattr+0x76/0x350 [cachefiles]
cachefiles_acquire_volume+0x2be/0x500 [cachefiles]
fscache_create_volume_work+0x68/0x160 [fscache]
process_one_work+0x3b7/0x6a0
worker_thread+0x2c4/0x650
kthread+0x16c/0x1a0
ret_from_fork+0x22/0x30

The buggy address belongs to the object at ffff888101e02af0
which belongs to the cache kmalloc-8 of size 8
The buggy address is located 4 bytes inside of
8-byte region [ffff888101e02af0, ffff888101e02af8)

The buggy address belongs to the physical page:
page:00000000a2292d70 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x101e02
flags: 0x17ffffc0000200(slab|node=0|zone=2|lastcpupid=0x1fffff)
raw: 0017ffffc0000200 0000000000000000 dead000000000001 ffff888100042280
raw: 0000000000000000 0000000080660066 00000001ffffffff 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff888101e02980: fc 00 fc fc fc fc 00 fc fc fc fc 00 fc fc fc fc
ffff888101e02a00: 00 fc fc fc fc 00 fc fc fc fc 00 fc fc fc fc 00
>ffff888101e02a80: fc fc fc fc 00 fc fc fc fc 00 fc fc fc fc 04 fc
^
ffff888101e02b00: fc fc fc 00 fc fc fc fc 00 fc fc fc fc 00 fc fc
ffff888101e02b80: fc fc 00 fc fc fc fc 00 fc fc fc fc 00 fc fc fc
==================================================================