affs: struct slink_front: Replace 1-element array with flexible array

Replace the deprecated[1] use of a 1-element array in
struct slink_front with a modern flexible array.

No binary differences are present after this conversion.

Link: https://github.com/KSPP/linux/issues/79

 [1]
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Kees Cook <kees@kernel.org>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>