commit 4fac6269172eab2be0490a12936248113d14848d
Author: Jan Lindström <jan.lindstrom@mariadb.com>
Date: Mon Jun 29 12:48:42 2015 +0300

MDEV-8390: innodb.innodb-flush-changed-page-bitmaps crashes

Analysis: Flush changed page bitmaps calls log_online_follow_redo_log
that later calls log_group_read_log_seg with release_mutex = true
and that causes the log_sys mutex being released before
log_decrypt_after_read function that later required log_sys mutex.

Fix: Enter log_sys mutex before log_decrypt_after_read if mutex
is released and release the mutex after the function if we
acquired it before function call.