Fix a memory leak where Galois field acceleration functions created by
ErasureCodeJerasure::prepare() were never freed. ASan detected this as
a one-time leak when the plugin was unloaded.

Add jerasure_finish() destructor function in jerasure_init.cc to free
the allocated Galois field operations. Since jerasure_init.cc and
galois.c are built into the same object library, jerasure_finish() can
access and clean up the global static acceleration functions defined
in galois.c.

The destructor function is automatically called when the shared library
(plugin) is unloaded, ensuring proper cleanup without requiring explicit
calls from client code.

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