IB/mthca: Add missing sg_init_table() in mthca_map_user_db()

Usually harmless, since the scatterlist is always hard-coded to a length
of 1, but it triggers a BUG() if CONFIG_DEBUG_SG=y, so we better fix it.
This fixes <http://bugzilla.kernel.org/show_bug.cgi?id=9934>.
Signed-off-by: Roland Dreier <rolandd@cisco.com>

drivers/infiniband/hw/mthca/mthca_memfree.c

@@ -542,6 +542,7 @@ struct mthca_user_db_table *mthca_init_user_db_tab(struct mthca_dev *dev)
 	for (i = 0; i < npages; ++i) {
 		db_tab->page[i].refcount = 0;
 		db_tab->page[i].uvirt    = 0;
+		sg_init_table(&db_tab->page[i].mem, 1);
 	}
 
 	return db_tab;