scsi: target: tcmu: Annotate struct tcmu_tmr with __counted_by

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS
(for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

As found with Coccinelle[1], add __counted_by for struct tcmu_tmr.

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci

Cc: Bodo Stroesser <bostroesser@gmail.com>
Cc: "Martin K. Petersen" <martin.petersen@oracle.com>
Cc: linux-scsi@vger.kernel.org
Cc: target-devel@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20230922175300.work.148-kees@kernel.org
Reviewed-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Reviewed-by: Bodo Stroesser <bostroesser@gmail.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Author: kees

drivers/target/target_core_user.c

@@ -201,7 +201,7 @@ struct tcmu_tmr {
 
 	uint8_t tmr_type;
 	uint32_t tmr_cmd_cnt;
-	int16_t tmr_cmd_ids[];
+	int16_t tmr_cmd_ids[] __counted_by(tmr_cmd_cnt);
 };
 
 /*