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

fil_space_destroy_crypt_data(): Call mutex_free() to pair with
fil_space_create_crypt_data() and fil_space_read_crypt_data().

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

fil_space_free_low(): Invoke fil_space_destroy_crypt_data().

fil_close(): Invoke fil_space_crypt_cleanup(), just like
fil_init() invoked fil_space_crypt_init().

Datafile::shutdown(): Set m_crypt_info=NULL without dereferencing
the pointer. The object will be freed along with the fil_space_t
in fil_space_free_low().
Remove some unnecessary conditions (ut_free(NULL) is OK).

srv_shutdown_all_bg_threads(): Shut down the encryption threads
by calling fil_crypt_threads_end().

srv_shutdown_bg_undo_sources(): Do not prematurely call
fil_crypt_threads_end(). Many pages can still be written by
change buffer merge, rollback of incomplete transactions, and
purge, especially in slow shutdown (innodb_fast_shutdown=0).

innobase_shutdown_for_mysql(): Call fil_crypt_threads_cleanup()
also when innodb_read_only=1, because the threads will have been
created also in that case.

sync_check_close(): Re-enable the invocation of sync_latch_meta_destroy()
to free the mutex list.