scsi: libsas: Introduce struct smp_disc_resp

scsi: libsas: Introduce struct smp_disc_resp

When compiling with gcc 12, several warnings are thrown by gcc when
compiling drivers/scsi/libsas/sas_expander.c, e.g.:

In function ‘sas_get_phy_change_count’,
    inlined from ‘sas_find_bcast_phy.constprop’ at
drivers/scsi/libsas/sas_expander.c:1737:9:
drivers/scsi/libsas/sas_expander.c:1697:39: warning: array subscript
‘struct smp_resp[0]’ is partly outside array bounds of ‘unsigned
char[56]’ [-Warray-bounds]
 1697 |                 *pcc = disc_resp->disc.change_count;
      |                        ~~~~~~~~~~~~~~~^~~~~~~~~~~~~

This is due to the use of the struct smp_resp to aggregate all possible
response types using a union but allocating a response buffer with a size
exactly equal to the size of the response type needed. This leads to access
to fields of struct smp_resp from an allocated memory area that is smaller
than the size of struct smp_resp.

Fix this by defining struct smp_disc_resp for sas discovery operations.
Since this structure and the generic struct smp_resp are identical for
the little endian and big endian archs, move the definition of these
structures at the end of include/scsi/sas.h to avoid repeating their
definition.

Link: https://lore.kernel.org/r/20220609022456.409087-2-damien.lemoal@opensource.wdc.com
Reviewed-by: John Garry <john.garry@huawei.com>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>