IB/mthca: Use memset_startat() for clearing mpt_entry

In preparation for FORTIFY_SOURCE performing compile-time and run-time
field bounds checking for memset(), avoid intentionally writing across
neighboring fields.

Use memset_startat() so memset() doesn't get confused about writing beyond
the destination member that is intended to be the starting point of
zeroing through the end of the struct.

Link: https://lore.kernel.org/r/20211213223331.135412-15-keescook@chromium.orgSigned-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/infiniband/hw/mthca/mthca_mr.c

@@ -467,8 +467,7 @@ int mthca_mr_alloc(struct mthca_dev *dev, u32 pd, int buffer_size_shift,
 	mpt_entry->start     = cpu_to_be64(iova);
 	mpt_entry->length    = cpu_to_be64(total_size);
 
-	memset(&mpt_entry->lkey, 0,
-	       sizeof *mpt_entry - offsetof(struct mthca_mpt_entry, lkey));
+	memset_startat(mpt_entry, 0, lkey);
 
 	if (mr->mtt)
 		mpt_entry->mtt_seg =