RDMA/siw: 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 siw_pbl.

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 7649773 ("cxgb3/l2t: Fix undefined behaviour")

Link: https://lore.kernel.org/r/20200519233018.GA6105@embeddedor
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/sw/siw/siw.h

@@ -139,7 +139,7 @@ struct siw_pble {
 struct siw_pbl {
 	unsigned int num_buf;
 	unsigned int max_buf;
-	struct siw_pble pbe[1];
+	struct siw_pble pbe[];
 };
 
 /*

drivers/infiniband/sw/siw/siw_mem.c

@@ -349,14 +349,11 @@ dma_addr_t siw_pbl_get_buffer(struct siw_pbl *pbl, u64 off, int *len, int *idx)
 struct siw_pbl *siw_pbl_alloc(u32 num_buf)
 {
 	struct siw_pbl *pbl;
-	int buf_size = sizeof(*pbl);
 
 	if (num_buf == 0)
 		return ERR_PTR(-EINVAL);
 
-	buf_size += ((num_buf - 1) * sizeof(struct siw_pble));
-
-	pbl = kzalloc(buf_size, GFP_KERNEL);
+	pbl = kzalloc(struct_size(pbl, pbe, num_buf), GFP_KERNEL);
 	if (!pbl)
 		return ERR_PTR(-ENOMEM);