Make some global fil_crypt_ variables static.

fil_close(): Call mutex_free(&fil_system->mutex) also in InnoDB, not
only in XtraDB. In InnoDB, sync_close() was called before fil_close().

innobase_shutdown_for_mysql(): Call fil_close() before sync_close(),
similar to XtraDB shutdown.

fil_space_crypt_cleanup(): Call mutex_free() to pair with
fil_space_crypt_init().

fil_crypt_threads_cleanup(): Call mutex_free() to pair with
fil_crypt_threads_init().