MDEV-27774 fixup: log_sys.mutex is log_sys.latch

storage/innobase/include/log0log.h

@@ -278,7 +278,7 @@ typedef srw_lock log_rwlock_t;
 					new query step is started */
   /** latest completed checkpoint (protected by latch.wr_lock()) */
   Atomic_relaxed<lsn_t> last_checkpoint_lsn;
-  /** next checkpoint LSN (protected by log_sys.mutex) */
+  /** next checkpoint LSN (protected by log_sys.latch) */
   lsn_t next_checkpoint_lsn;
   /** next checkpoint number (protected by latch.wr_lock()) */
   ulint next_checkpoint_no;

storage/innobase/mtr/mtr0mtr.cc

@@ -997,7 +997,7 @@ bool mtr_t::commit_file(fil_space_t &space, const char *name)
 /** Commit a mini-transaction that did not modify any pages,
 but generated some redo log on a higher level, such as
 FILE_MODIFY records and an optional FILE_CHECKPOINT marker.
-The caller must hold log_sys.mutex.
+The caller must hold exclusive log_sys.latch.
 This is to be used at log_checkpoint().
 @param checkpoint_lsn   the log sequence number of a checkpoint, or 0
 @return current LSN */