afs: Fix the afs.cell and afs.volume xattr handlers

afs: Fix the afs.cell and afs.volume xattr handlers

BugLink: https://bugs.launchpad.net/bugs/1863019
[ Upstream commit c73aa4102f5b9f261a907c3b3df94cd2c478504d ]

Fix the ->get handlers for the afs.cell and afs.volume xattrs to pass the
source data size to memcpy() rather than target buffer size.

Overcopying the source data occasionally causes the kernel to oops.

Fixes: d3e3b7eac886 ("afs: Add metadata xattrs")
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>