Handle extending writes correctly in NFS proxy

Previously, multiple parallel writes extending file size through NFS were
racing with each other and triggering deletions of part of the written data

I.e. if you mounted vitastor-nfs and just copied a file into it in MC then
you could end up with only a part of the file actually written

src/nfs_conn.cpp

@@ -446,6 +446,7 @@ static void nfs_resize_write(nfs_client_t *self, rpc_op_t *rop, uint64_t inode,
         self->parent->cmd->loop_and_wait(self->parent->cmd->start_modify(json11::Json::object {
             { "image", inode_it->second.name },
             { "resize", new_size },
+            { "inc_size", true },
             { "force_size", true },
         }), [=](const cli_result_t & r)
         {