scsi: mpi3mr: Replace 1-element array with flex-array

scsi: mpi3mr: Replace 1-element array with flex-array

Nothing else defined MPI3_NVME_ENCAP_CMD_MAX, so the "command" buffer was
being defined as a fake flexible array of size 1. Replace this with a
proper flex array. Avoids this GCC 13 warning under -fstrict-flex-arrays=3:

In function 'fortify_memset_chk',
    inlined from 'mpi3mr_build_nvme_sgl' at ../drivers/scsi/mpi3mr/mpi3mr_app.c:693:2,
    inlined from 'mpi3mr_bsg_process_mpt_cmds.constprop' at ../drivers/scsi/mpi3mr/mpi3mr_app.c:1214:8:
../include/linux/fortify-string.h:430:25: warning: call to '__write_overflow_field' declared with attribute warning: detected write beyond size of field (1st parameter); maybe use struct_group()?  [-Wattribute-warning]
  430 |                         __write_overflow_field(p_size_field, size);
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Link: https://lore.kernel.org/r/20230204183715.never.937-kees@kernel.org
Cc: Sumit Saxena <sumit.saxena@broadcom.com>
Cc: Himanshu Madhani <himanshu.madhani@oracle.com>
Cc: "Martin K. Petersen" <martin.petersen@oracle.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: kernel test robot <lkp@intel.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>