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/[e-mail]
Cc: Sumit Saxena <[e-mail]>
Cc: Himanshu Madhani <[e-mail]>
Cc: "Martin K. Petersen" <[e-mail]>
Cc: Stephen Rothwell <[e-mail]>
Cc: kernel test robot <[e-mail]>
Signed-off-by: Kees Cook <[e-mail]>
Signed-off-by: Martin K. Petersen <[e-mail]>

include/uapi/scsi/scsi_bsg_mpi3mr.h

···
 	} cmd;
 };
 
-
-/* MPI3: NVMe Encasulation related definitions */
-#ifndef MPI3_NVME_ENCAP_CMD_MAX
-#define MPI3_NVME_ENCAP_CMD_MAX               (1)
-#endif
-
 struct mpi3_nvme_encapsulated_request {
 	__le16	host_tag;
 	__u8	ioc_use_only02;
···
 	__le16	flags;
 	__le32	data_length;
 	__le32  reserved14[3];
-	__le32	command[MPI3_NVME_ENCAP_CMD_MAX];
+	__le32	command[];
 };
 
 struct mpi3_nvme_encapsulated_error_reply {