drm/amdgpu/smu10: Replace one-element array and use struct_size() helper

The current codebase makes use of one-element arrays in the following
form:

struct something {
    int length;
    u8 data[1];
};

struct something *instance;

instance = kmalloc(sizeof(*instance) + size, GFP_KERNEL);
instance->length = size;
memcpy(instance->data, source, size);

but the preferred mechanism to declare variable-length types such as
these ones is a flexible array member[1][2], introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on. So, replace
the one-element array with a flexible-array member.

Also, make use of the new struct_size() helper to properly calculate the
size of struct smu10_voltage_dependency_table.

This issue was found with the help of Coccinelle and, audited and fixed
_manually_.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732 ("cxgb3/l2t: Fix undefined behaviour")
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/powerplay/hwmgr/smu10_hwmgr.c

@@ -410,12 +410,10 @@ static int smu10_get_clock_voltage_dependency_table(struct pp_hwmgr *hwmgr,
 			struct smu10_voltage_dependency_table **pptable,
 			uint32_t num_entry, const DpmClock_t *pclk_dependency_table)
 {
-	uint32_t table_size, i;
+	uint32_t i;
 	struct smu10_voltage_dependency_table *ptable;
 
-	table_size = sizeof(uint32_t) + sizeof(struct smu10_voltage_dependency_table) * num_entry;
-	ptable = kzalloc(table_size, GFP_KERNEL);
-
+	ptable = kzalloc(struct_size(ptable, entries, num_entry), GFP_KERNEL);
 	if (NULL == ptable)
 		return -ENOMEM;
 

drivers/gpu/drm/amd/powerplay/hwmgr/smu10_hwmgr.h

@@ -192,7 +192,7 @@ struct smu10_clock_voltage_dependency_record {
 
 struct smu10_voltage_dependency_table {
 	uint32_t count;
-	struct smu10_clock_voltage_dependency_record entries[1];
+	struct smu10_clock_voltage_dependency_record entries[];
 };
 
 struct smu10_clock_voltage_information {