storage: fix local_call not finding C stored procedures

Currently, if function was created as C stored procedure with
box.schema.func.create, all types of router.call and router.map_callrw
cannot find it and return `Procedure 'name' is not defined` error.
This is cased by the fact that both of these function use local_call,
which invokes net_box.self.call, which doesn't currently work with
C stored procedures due to the bug.

Let's use box.func, where it's possible instead of net_box.self.call
in local_call.

Closes #436

NO_DOC=bugfix
NO_TEST=<already tested>

example/storage.lua

@@ -6,6 +6,7 @@ require('strict').on()
 local fio = require('fio')
 local NAME = fio.basename(arg[0], '.lua')
 local fiber = require('fiber')
+local multlib = require('multlib')
 
 -- Check if we are running under test-run
 if os.getenv('ADMIN') then
@@ -53,6 +54,9 @@ box.once("testapp:schema:1", function()
     account:create_index('bucket_id', {parts = {'bucket_id'}, unique = false})
     box.snapshot()
 
+    box.schema.func.create('multlib.mult50', {language = 'C'})
+    box.schema.role.grant('public', 'execute', 'function', 'multlib.mult50')
+
     box.schema.func.create('customer_lookup')
     box.schema.role.grant('public', 'execute', 'function', 'customer_lookup')
     box.schema.func.create('customer_add')

vshard/storage/init.lua

@@ -251,7 +251,16 @@ end
 -- exceptions are not allowed.
 --
 local function local_call(func_name, args)
-    return pcall(netbox_self_call, netbox_self, func_name, args)
+    -- box.func was introduced only in Tarantool 2.2. net_box.self.call
+    -- doesn't work with C stored procedures up to Tarantool 3.0.0-alpha3.
+    -- If function wasn't created with box.schema.func interface, then it's
+    -- not in box.func and we should still use net_box.self.call.
+    if box.func == nil or box.func[func_name] ==  nil then
+        return pcall(netbox_self_call, netbox_self, func_name, args)
+    end
+
+    local func = box.func[func_name]
+    return pcall(func.call, func, args)
 end
 
 --