smb: client: fix zero rsize error messages

cifs_prepare_read() might be called with a disconnected channel, where
TCP_Server_Info::max_read is set to zero due to reconnect, so calling
->negotiate_rize() will set @rsize to default min IO size (64KiB) and
then logging

	CIFS: VFS: SMB: Zero rsize calculated, using minimum value
	65536

If the reconnect happens in cifsd thread, cifs_renegotiate_iosize()
will end up being called and then @rsize set to the expected value.

Since we can't rely on the value of @server->max_read by the time we
call cifs_prepare_read(), try to ->negotiate_rize() only if
@cifs_sb->ctx->rsize is zero.

Reported-by: Steve French <stfrench@microsoft.com>
Fixes: c59f7c96 ("smb: client: ensure aligned IO sizes")
Signed-off-by: Paulo Alcantara (Red Hat) <pc@manguebit.com>
Signed-off-by: Steve French <stfrench@microsoft.com>