fil_crypt_thread(): Do invoke fil_crypt_complete_rotate_space()
when the tablespace is about to be dropped. Also, remove a redundant
check whether rotate_thread_t::space is NULL. It can only become
NULL when fil_crypt_find_space_to_rotate() returns false, and in
that case we would already have terminated the loop.

fil_crypt_find_page_to_rotate(): Remove a redundant check for
space->crypt_data == NULL. Once encryption metadata has been
created for a tablespace, it cannot be removed without dropping
the entire tablespace.