RDMA/uverbs: Use offsetofend instead of opencoding

Discovered this was available already.
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>

drivers/infiniband/core/uverbs_cmd.c

@@ -3708,9 +3708,6 @@ static int ib_uverbs_ex_modify_cq(struct uverbs_attr_bundle *attrs)
  * trailing driver_data flex array. In this case the size of the base struct
  * cannot be changed.
  */
-#define offsetof_after(_struct, _member)                                       \
-	(offsetof(_struct, _member) + sizeof(((_struct *)NULL)->_member))
-
 #define UAPI_DEF_WRITE_IO(req, resp)                                           \
 	.write.has_resp = 1 +                                                  \
 			  BUILD_BUG_ON_ZERO(offsetof(req, response) != 0) +    \
@@ -3741,11 +3738,11 @@ static int ib_uverbs_ex_modify_cq(struct uverbs_attr_bundle *attrs)
  */
 #define UAPI_DEF_WRITE_IO_EX(req, req_last_member, resp, resp_last_member)     \
 	.write.has_resp = 1,                                                   \
-	.write.req_size = offsetof_after(req, req_last_member),                \
-	.write.resp_size = offsetof_after(resp, resp_last_member)
+	.write.req_size = offsetofend(req, req_last_member),                   \
+	.write.resp_size = offsetofend(resp, resp_last_member)
 
 #define UAPI_DEF_WRITE_I_EX(req, req_last_member)                              \
-	.write.req_size = offsetof_after(req, req_last_member)
+	.write.req_size = offsetofend(req, req_last_member)
 
 const struct uapi_definition uverbs_def_write_intf[] = {
 	DECLARE_UVERBS_OBJECT(