drbd: 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 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

---
 drivers/block/drbd/drbd_int.h      |    2 +-
 drivers/block/drbd/drbd_protocol.h |    8 ++++----
 2 files changed, 5 insertions(+), 5 deletions(-)

index aae99a2d7bd4..a3314dd781a7 100644
@@ -620,7 +620,7 @@ struct fifo_buffer {
 	unsigned int head_index;
 	unsigned int size;
 	int total; /* sum of all values */
-	int values[0];
+	int values[];
 };
 extern struct fifo_buffer *fifo_alloc(unsigned int fifo_size);
 

index e6fc5ad72501..dea59c92ecc1 100644
@@ -271,7 +271,7 @@ struct p_rs_param {
 	u32 resync_rate;
 
 	      /* Since protocol version 88 and higher. */
-	char verify_alg[0];
+	char verify_alg[];
 } __packed;
 
 struct p_rs_param_89 {

@@ -305,7 +305,7 @@ struct p_protocol {
 	u32 two_primaries;
 
 	/* Since protocol version 87 and higher. */
-	char integrity_alg[0];
+	char integrity_alg[];
 
 } __packed;
 

@@ -360,7 +360,7 @@ struct p_sizes {
 	u16	    dds_flags; /* use enum dds_flags here. */
 
 	/* optional queue_limits if (agreed_features & DRBD_FF_WSAME) */
-	struct o_qlim qlim[0];
+	struct o_qlim qlim[];
 } __packed;
 
 struct p_state {

@@ -409,7 +409,7 @@ struct p_compressed_bm {
 	 */
 	u8 encoding;
 
-	u8 code[0];
+	u8 code[];
 } __packed;
 
 struct p_delay_probe93 {