RDMA/uverbs: check for allocation failure in uapi_add_elm()

If the kzalloc() fails then we should return ERR_PTR(-ENOMEM).  In the
current code it's possible that the kzalloc() fails and the
radix_tree_insert() inserts the NULL pointer successfully and we return
the NULL "elm" pointer to the caller.  That results in a NULL pointer
dereference.

Fixes: 9ed3e5f ("IB/uverbs: Build the specs into a radix tree at runtime")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/core/uverbs_uapi.c

@@ -22,6 +22,8 @@ static void *uapi_add_elm(struct uverbs_api *uapi, u32 key, size_t alloc_size)
 		return ERR_PTR(-EOVERFLOW);
 
 	elm = kzalloc(alloc_size, GFP_KERNEL);
+	if (!elm)
+		return ERR_PTR(-ENOMEM);
 	rc = radix_tree_insert(&uapi->radix, key, elm);
 	if (rc) {
 		kfree(elm);