Updated locking documentation for transaction_t

Some members of transaction_t are allowed to be read without any lock
being held if accessed from the correct context.  We used LockDoc's
findings to determine those members.  Each member of them is marked
with a short comment: "no lock needed for jbd2 thread".
Signed-off-by: Alexander Lochmann <alexander.lochmann@tu-dortmund.de>
Signed-off-by: Horst Schirmeier <horst.schirmeier@tu-dortmund.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20210211171410.17984-1-alexander.lochmann@tu-dortmund.deSigned-off-by: Theodore Ts'o <tytso@mit.edu>

include/linux/jbd2.h

@@ -594,18 +594,22 @@ struct transaction_s
 	 */
 	unsigned long		t_log_start;
 
-	/* Number of buffers on the t_buffers list [j_list_lock] */
+	/* 
+	 * Number of buffers on the t_buffers list [j_list_lock, no locks
+	 * needed for jbd2 thread]
+	 */
 	int			t_nr_buffers;
 
 	/*
 	 * Doubly-linked circular list of all buffers reserved but not yet
-	 * modified by this transaction [j_list_lock]
+	 * modified by this transaction [j_list_lock, no locks needed fo
+	 * jbd2 thread]
 	 */
 	struct journal_head	*t_reserved_list;
 
 	/*
 	 * Doubly-linked circular list of all metadata buffers owned by this
-	 * transaction [j_list_lock]
+	 * transaction [j_list_lock, no locks needed for jbd2 thread]
 	 */
 	struct journal_head	*t_buffers;
 
@@ -629,9 +633,11 @@ struct transaction_s
 	struct journal_head	*t_checkpoint_io_list;
 
 	/*
-	 * Doubly-linked circular list of metadata buffers being shadowed by log
-	 * IO.  The IO buffers on the iobuf list and the shadow buffers on this
-	 * list match each other one for one at all times. [j_list_lock]
+	 * Doubly-linked circular list of metadata buffers being
+	 * shadowed by log IO.  The IO buffers on the iobuf list and
+	 * the shadow buffers on this list match each other one for
+	 * one at all times. [j_list_lock, no locks needed for jbd2
+	 * thread]
 	 */
 	struct journal_head	*t_shadow_list;