fil_tablespace_iterate(): Call fil_space_destroy_crypt_data() to
invoke mutex_free() for the mutex_create() that was done in
fil_space_read_crypt_data(). Also, remember to free
iter.crypt_io_buffer.

The failure to call mutex_free() would cause sync_latch_meta_destroy()
to access freed memory on shutdown. This affected the IMPORT of
encrypted tablespaces.