Lock inodes in Client::send_request and build_client_request.

Note lock order is client_lock, then inode locks (the client_lock
may not be taken if any inode locks are already held).

Planned order for taking multiple inode locks is in address order.
The code does not yet do this.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>
Lock inodes in Client::handle_client_reply.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>