scsi: aacraid: Replace one-element array with flexible-array member

There is a regular need in the kernel to provide a way to declare having a
dynamically sized set of trailing elements in a structure. Kernel code
should always use “flexible array members”[1] for these cases. The older
style of one-element or zero-length arrays should no longer be used[2].

Refactor the code according to the use of a flexible-array member in struct
aac_raw_io2 instead of one-element array, and use the struct_size() helper.

Also, this helps with the ongoing efforts to enable -Warray-bounds by
fixing the following warnings:

drivers/scsi/aacraid/aachba.c: In function ‘aac_build_sgraw2’:
drivers/scsi/aacraid/aachba.c:3970:18: warning: array subscript 1 is above array bounds of ‘struct sge_ieee1212[1]’ [-Warray-bounds]
 3970 |     if (rio2->sge[j].length % (i*PAGE_SIZE)) {
      |         ~~~~~~~~~^~~
drivers/scsi/aacraid/aachba.c:3974:27: warning: array subscript 1 is above array bounds of ‘struct sge_ieee1212[1]’ [-Warray-bounds]
 3974 |     nseg_new += (rio2->sge[j].length / (i*PAGE_SIZE));
      |                  ~~~~~~~~~^~~
drivers/scsi/aacraid/aachba.c:4011:28: warning: array subscript 1 is above array bounds of ‘struct sge_ieee1212[1]’ [-Warray-bounds]
 4011 |   for (j = 0; j < rio2->sge[i].length / (pages * PAGE_SIZE); ++j) {
      |                   ~~~~~~~~~^~~
drivers/scsi/aacraid/aachba.c:4012:24: warning: array subscript 1 is above array bounds of ‘struct sge_ieee1212[1]’ [-Warray-bounds]
 4012 |    addr_low = rio2->sge[i].addrLow + j * pages * PAGE_SIZE;
      |               ~~~~~~~~~^~~
drivers/scsi/aacraid/aachba.c:4014:33: warning: array subscript 1 is above array bounds of ‘struct sge_ieee1212[1]’ [-Warray-bounds]
 4014 |    sge[pos].addrHigh = rio2->sge[i].addrHigh;
      |                        ~~~~~~~~~^~~
drivers/scsi/aacraid/aachba.c:4015:28: warning: array subscript 1 is above array bounds of ‘struct sge_ieee1212[1]’ [-Warray-bounds]
 4015 |    if (addr_low < rio2->sge[i].addrLow)
      |                   ~~~~~~~~~^~~

[1] https://en.wikipedia.org/wiki/Flexible_array_member
[2] https://www.kernel.org/doc/html/v5.9/process/deprecated.html#zero-length-and-one-element-arrays

Link: https://github.com/KSPP/linux/issues/79
Link: https://github.com/KSPP/linux/issues/109
Link: https://lore.kernel.org/lkml/60414244.ur4%2FkI+fBF1ohKZs%25lkp@intel.com/
Link: https://lore.kernel.org/r/20210421185611.GA105224@embeddedor


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Build-tested-by: kernel test robot <lkp@intel.com>