cachefiles: Fix potential UAF/KASAN warning

Currently, trace_cachefiles_coherency() is being passed a pointer to a
__be64 lain over the coherency data in struct cachefiles_xattr so that it
can display the first 8 bytes.  However, the data is of variable length and
could even be 0 bytes.  This could lead to a UAF or KASAN warning.

Fix this by making sure the buffer has room for at least 8 bytes and that
those 8 bytes are pre-cleared.

Further, those bytes are not 8-byte aligned, so fix the tracepoint to
extract the data as four 2-byte words (they are 2-byte aligned) and
reassemble the __be64.  The compiler will convert this into a single 8-byte
load where the CPU supports it.

Fixes: 229105e5cfd9 ("cachefiles: Add auxiliary data trace")
Link: https://sashiko.dev/#/patchset/20260810144746.574036-1-dhowells%40redhat.com
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Paulo Alcantara <pc@manguebit.org>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org