scsi: target: uapi: Replace fake flex-array with flexible-array member

Zero-length arrays as fake flexible arrays are deprecated and we are moving
towards adopting C99 flexible-array members instead.

Address the following warning found with GCC-13 and -fstrict-flex-arrays=3
enabled:

CC      drivers/target/target_core_user.o
drivers/target/target_core_user.c: In function ‘queue_cmd_ring’:
drivers/target/target_core_user.c:1096:15: warning: array subscript 0 is outside array bounds of ‘struct iovec[0]’ [-Warray-bounds=]
 1096 |         iov = &entry->req.iov[0];
      |               ^~~~~~~~~~~~~~~~~~
In file included from drivers/target/target_core_user.c:31:
./include/uapi/linux/target_core_user.h:122:38: note: while referencing ‘iov’
  122 |                         struct iovec iov[0];
      |                                      ^~~

This helps with the ongoing efforts to tighten the FORTIFY_SOURCE routines
on memcpy() and help us make progress towards globally enabling
-fstrict-flex-arrays=3 [1].

Link: https://github.com/KSPP/linux/issues/21
Link: https://github.com/KSPP/linux/issues/270
Link: https://gcc.gnu.org/pipermail/gcc-patches/2022-October/602902.html [1]
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/ZBSchMvTdl7VObKI@work
Reviewed-by: Bodo Stroesser <bostroesser@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

include/uapi/linux/target_core_user.h

@@ -119,7 +119,7 @@ struct tcmu_cmd_entry {
 			__u64 cdb_off;
 			__u64 __pad1;
 			__u64 __pad2;
-			struct iovec iov[0];
+			__DECLARE_FLEX_ARRAY(struct iovec, iov);
 		} req;
 		struct {
 			__u8 scsi_status;