MDEV-28314 : The Galera cluster primary node goes into hang mode when innodb_encryption_threads is enabled

When we enable writes after Galera SST srv_n_fil_crypt_threads needs
to be set temporally to 0 (as was done when writes were disabled)
to make sure that encryption threads will be really started based
on old value of encryption threads.

Fix provided by Marko Mäkelä.