remoteproc: Replace zero-length array with flexible-array

The current codebase makes use of the zero-length array language
extension to the C90 standard, 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.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

sizeof(flexible-array-member) triggers a warning because flexible array
members have incomplete type[1]. There are some instances of code in
which the sizeof operator is being incorrectly/erroneously applied to
zero-length arrays and the result is zero. Such instances may be hiding
some bugs. So, this work (flexible-array member conversions) will also
help to get completely rid of those sorts of issues.

This issue was found with the help of Coccinelle.

[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")

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20200507191943.GA16033@embeddedor
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>

include/linux/remoteproc.h

@@ -73,7 +73,7 @@ struct resource_table {
 	u32 ver;
 	u32 num;
 	u32 reserved[2];
-	u32 offset[0];
+	u32 offset[];
 } __packed;
 
 /**
@@ -87,7 +87,7 @@ struct resource_table {
  */
 struct fw_rsc_hdr {
 	u32 type;
-	u8 data[0];
+	u8 data[];
 } __packed;
 
 /**
@@ -306,7 +306,7 @@ struct fw_rsc_vdev {
 	u8 status;
 	u8 num_of_vrings;
 	u8 reserved[2];
-	struct fw_rsc_vdev_vring vring[0];
+	struct fw_rsc_vdev_vring vring[];
 } __packed;
 
 struct rproc;