nvme: 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 7649773293 ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>

drivers/nvme/host/fc.c

@@ -108,7 +108,7 @@ struct nvme_fc_fcp_op {
 struct nvme_fcp_op_w_sgl {
 	struct nvme_fc_fcp_op	op;
 	struct scatterlist	sgl[NVME_INLINE_SG_CNT];
-	uint8_t			priv[0];
+	uint8_t			priv[];
 };
 
 struct nvme_fc_lport {

drivers/nvme/host/lightnvm.c

@@ -171,7 +171,7 @@ struct nvme_nvm_bb_tbl {
 	__le32	tdresv;
 	__le32	thresv;
 	__le32	rsvd2[8];
-	__u8	blk[0];
+	__u8	blk[];
 };
 
 struct nvme_nvm_id20_addrf {

include/linux/nvme.h

@@ -1185,7 +1185,7 @@ struct nvmf_disc_rsp_page_hdr {
 	__le64		numrec;
 	__le16		recfmt;
 	__u8		resv14[1006];
-	struct nvmf_disc_rsp_page_entry entries[0];
+	struct nvmf_disc_rsp_page_entry entries[];
 };
 
 enum {