FROMLIST: afs: xattr: use scnprintf

sprintf and snprintf are fragile in future maintenance, switch to
using scnprintf to ensure no accidental Use After Free conditions
are introduced.

Signed-off-by: Mark Salyzyn <salyzyn@android.com>
Cc: linux-kernel@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org
Cc: David Howells <dhowells@redhat.com>
Cc: linux-afs@lists.infradead.org
Cc: Jan Kara <jack@suse.cz>

(cherry picked from https://lore.kernel.org/lkml/20191105154850.187723-1-salyzyn@android.com/

)
Signed-off-by: Mark Salyzyn <salyzyn@google.com>
Bug: 133515582
Bug: 136124883
Bug: 129319403
Change-Id: I6fca97f90f60472c1a7737898b4629b20c04c495