The most important addition is that a mutex is placed around the call to the system's crypt() function to ensure thread safety. This mutex is necessary to close a bunch of possible attacks on the libc crypt() function on multi threaded systems.

He goes on to explain why it's so important - correcting a possible race condition between competing threads looking to use the crypt() function at the same time. They use the same shared memory space and, because of this, can return possible invalid data. The patch changes this behavior and replaces it with a blowfish implementation to make things more thread-safe.