scsi: libsas: 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 76497732 ("cxgb3/l2t: Fix undefined behaviour")

Link: https://lore.kernel.org/r/20200507192147.GA16206@embeddedorReviewed-by: John Garry <john.garry@huawei.com>
Reviewed-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/aic94xx/aic94xx_sds.c

@@ -406,7 +406,7 @@ struct asd_manuf_sec {
 	u8    sas_addr[SAS_ADDR_SIZE];
 	u8    pcba_sn[ASD_PCBA_SN_SIZE];
 	/* Here start the other segments */
-	u8    linked_list[0];
+	u8    linked_list[];
 } __attribute__ ((packed));
 
 struct asd_manuf_phy_desc {
@@ -449,7 +449,7 @@ struct asd_ms_sb_desc {
 	u8    type;
 	u8    node_desc_index;
 	u8    conn_desc_index;
-	u8    _recvd[0];
+	u8    _recvd[];
 } __attribute__ ((packed));
 
 #if 0
@@ -478,12 +478,12 @@ struct asd_ms_conn_desc {
 	u8    size_sideband_desc;
 	u32   _resvd;
 	u8    name[16];
-	struct asd_ms_sb_desc sb_desc[0];
+	struct asd_ms_sb_desc sb_desc[];
 } __attribute__ ((packed));
 
 struct asd_nd_phy_desc {
 	u8    vp_attch_type;
-	u8    attch_specific[0];
+	u8    attch_specific[];
 } __attribute__ ((packed));
 
 #if 0
@@ -503,7 +503,7 @@ struct asd_ms_node_desc {
 	u8    size_phy_desc;
 	u8    _resvd;
 	u8    name[16];
-	struct asd_nd_phy_desc phy_desc[0];
+	struct asd_nd_phy_desc phy_desc[];
 } __attribute__ ((packed));
 
 struct asd_ms_conn_map {
@@ -518,7 +518,7 @@ struct asd_ms_conn_map {
 	u8    usage_model_id;
 	u32   _resvd;
 	struct asd_ms_conn_desc conn_desc[0];
-	struct asd_ms_node_desc node_desc[0];
+	struct asd_ms_node_desc node_desc[];
 } __attribute__ ((packed));
 
 struct asd_ctrla_phy_entry {
@@ -542,7 +542,7 @@ struct asd_ll_el {
 	u8   id0;
 	u8   id1;
 	__le16  next;
-	u8   something_here[0];
+	u8   something_here[];
 } __attribute__ ((packed));
 
 static int asd_poll_flash(struct asd_ha_struct *asd_ha)

include/scsi/sas.h

@@ -324,7 +324,7 @@ struct ssp_response_iu {
 	__be32 response_data_len;
 
 	u8     resp_data[0];
-	u8     sense_data[0];
+	u8     sense_data[];
 } __attribute__ ((packed));
 
 struct ssp_command_iu {
@@ -346,7 +346,7 @@ struct ssp_command_iu {
 	u8    add_cdb_len:6;
 
 	u8    cdb[16];
-	u8    add_cdb[0];
+	u8    add_cdb[];
 } __attribute__ ((packed));
 
 struct xfer_rdy_iu {
@@ -555,7 +555,7 @@ struct ssp_response_iu {
 	__be32 response_data_len;
 
 	u8     resp_data[0];
-	u8     sense_data[0];
+	u8     sense_data[];
 } __attribute__ ((packed));
 
 struct ssp_command_iu {
@@ -577,7 +577,7 @@ struct ssp_command_iu {
 	u8    _r_c:2;
 
 	u8    cdb[16];
-	u8    add_cdb[0];
+	u8    add_cdb[];
 } __attribute__ ((packed));
 
 struct xfer_rdy_iu {