[PATCH] IB/qib: Fix null pointer subtraction compiler warning

[Jason Gunthorpe <jgg@xxxxxxxxxx>]

>> drivers/infiniband/hw/qib/qib_sysfs.c:411:1: warning: performing pointer subtraction with a null pointer has undefined behavior
+[-Wnull-pointer-subtraction]
   QIB_DIAGC_ATTR(rc_resends);
   ^~~~~~~~~~~~~~~~~~~~~~~~~~
   drivers/infiniband/hw/qib/qib_sysfs.c:408:51: note: expanded from macro 'QIB_DIAGC_ATTR'
                   .counter = &((struct qib_ibport *)0)->rvp.n_##N - (u64 *)0,    \

Use offsetof and accomplish the type check using static_assert.

Fixes: 4a7aaf88c89f ("RDMA/qib: Use attributes for the port sysfs")
Reported-by: kernel test robot <lkp@xxxxxxxxx>
Signed-off-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
---
 drivers/infiniband/hw/qib/qib_sysfs.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/infiniband/hw/qib/qib_sysfs.c b/drivers/infiniband/hw/qib/qib_sysfs.c
index d57e49de6650be..452e2355d24eeb 100644
--- a/drivers/infiniband/hw/qib/qib_sysfs.c
+++ b/drivers/infiniband/hw/qib/qib_sysfs.c
@@ -403,9 +403,11 @@ static ssize_t diagc_attr_store(struct ib_device *ibdev, u32 port_num,
 }
 
 #define QIB_DIAGC_ATTR(N)                                                      \
+	static_assert(&((struct qib_ibport *)0)->rvp.n_##N != (u64 *)NULL);    \
 	static struct qib_diagc_attr qib_diagc_attr_##N = {                    \
 		.attr = __ATTR(N, 0664, diagc_attr_show, diagc_attr_store),    \
-		.counter = &((struct qib_ibport *)0)->rvp.n_##N - (u64 *)0,    \
+		.counter =                                                     \
+			offsetof(struct qib_ibport, rvp.n_##N) / sizeof(u64)   \
 	}
 
 QIB_DIAGC_ATTR(rc_resends);

base-commit: 6a217437f9f5482a3f6f2dc5fcd27cf0f62409ac
-- 
2.33.0