linux-user: Fix mmap_lock ordering

mmap_lock() can be called while tb_lock() is being held. To
avoid deadlock when one thread is holding mmap_lock and another
tb_lock, _always_ lock first tb_lock().

Signed-off-by: Riku Voipio <riku.voipio@nokia.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
