One-element arrays are deprecated, and we are replacing them with flexible
array members instead. So, replace one-element array with flexible-array
member in struct TAG_TW_New_Ioctl and refactor the rest of the code,
accordingly.

Notice that, in multiple places, the subtraction of 1 from
sizeof(TW_New_Ioctl) is removed, as this operation is now implicit
after the flex-array transformation.

Link: https://github.com/KSPP/linux/issues/79
Link: https://github.com/KSPP/linux/issues/206
Link: https://lore.kernel.org/r/YyyyvB30jnjRaw/F@workReviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>