affs: bound hash_pos before table lookup in affs_readdir

affs: bound hash_pos before table lookup in affs_readdir

affs_readdir() decodes ctx->pos into hash_pos and chain_pos and then
dereferences AFFS_HEAD(dir_bh)->table[hash_pos] before validating
that hash_pos is within the runtime table bound. Treat out-of-range
positions as end-of-directory before the first table lookup.

Signed-off-by: Hyungjung Joo <jhj140711@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>