cachefiles: Add auxiliary data trace

Add a display of the first 8 bytes of the downloaded auxiliary data and of
the on-disk stored auxiliary data as these are used in coherency
management.  In the case of afs, this holds the data version number.

Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/20241216204124.3752367-17-dhowells@redhat.com


cc: Jeff Layton <jlayton@kernel.org>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>