IB/qib: Fix clang confusion of NULL pointer comparison

clang becomes confused due to the comparison to NULL in a integer constant
expression context:

 >> drivers/infiniband/hw/qib/qib_sysfs.c:413:1: error: static_assert expression is not an integral constant expression
    QIB_DIAGC_ATTR(rc_resends);
    ^~~~~~~~~~~~~~~~~~~~~~~~~~
    drivers/infiniband/hw/qib/qib_sysfs.c:406:16: note: expanded from macro 'QIB_DIAGC_ATTR'
            static_assert(&((struct qib_ibport *)0)->rvp.n_##N != (u64 *)NULL);    \

Nathan found __same_type that solves this problem nicely, so use it instead.
Reported-by: kernel test robot <lkp@intel.com>
Suggested-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/infiniband/hw/qib/qib_sysfs.c

@@ -403,7 +403,7 @@ 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_assert(__same_type(((struct qib_ibport *)0)->rvp.n_##N, u64));  \
 	static struct qib_diagc_attr qib_diagc_attr_##N = {                    \
 		.attr = __ATTR(N, 0664, diagc_attr_show, diagc_attr_store),    \
 		.counter =                                                     \