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

AFFS uses struct affs_head's "table" array as a flexible array. Switch
this to a proper flexible array[1]. There are no sizeof() uses; struct
affs_head is only ever uses via direct casts. No binary output
differences were found after this change.

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>