jbd2: reduce journal_head size

Remove unused t_cow_tid field (ext4 copy-on-write support doesn't seem
to be happening) and change b_modified and b_jlist to bitfields thus
saving 8 bytes in the structure.
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: Zheng Liu <wenqing.lz@taobao.com>

include/linux/journal-head.h

@@ -31,21 +31,14 @@ struct journal_head {
 	/*
 	 * Journalling list for this buffer [jbd_lock_bh_state()]
 	 */
-	unsigned b_jlist;
+	unsigned b_jlist:4;
 
 	/*
 	 * This flag signals the buffer has been modified by
 	 * the currently running transaction
 	 * [jbd_lock_bh_state()]
 	 */
-	unsigned b_modified;
-
-	/*
-	 * This feild tracks the last transaction id in which this buffer
-	 * has been cowed
-	 * [jbd_lock_bh_state()]
-	 */
-	tid_t b_cow_tid;
+	unsigned b_modified:1;
 
 	/*
 	 * Copy of the buffer data frozen for writing to the log.