Problem was that log_scrub function did not take required log_sys mutex.
Background: Unused space in log blocks are padded with MLOG_DUMMY_RECORD if innodb-scrub-log
is enabled. As log files are written on circular fashion old log blocks can be reused
later for new redo-log entries. Scrubbing pads unused space in log blocks to avoid visibility
of the possible old redo-log contents.
log_scrub(): Take log_sys mutex
log_pad_current_log_block(): Increase srv_stats.n_log_scrubs if padding is done.
srv0srv.cc: Set srv_stats.n_log_scrubs to export vars innodb_scrub_log
ha_innodb.cc: Export innodb_scrub_log to global status.
