There is a race condition in InnoDB startup. A number of
fil_crypt_thread are created by fil_crypt_threads_init(). These threads
may call btr_scrub_complete_space() before btr_scrub_init() was called.
Those too early calls would be accessing an uninitialized scrub_stat_mutex.

innobase_start_or_create_for_mysql(): Invoke btr_scrub_init() before
fil_crypt_threads_init().

fil_crypt_complete_rotate_space(): Only invoke btr_scrub_complete_space()
if scrubbing is enabled. There is no need to update the statistics if
it is not enabled.