jbd: change the field "b_cow_tid" of struct journal_head from type unsigned to tid_t

In the definition of struct journal_head, the comment for
the field "unsigned b_cow_tid" says the field tracks the
last transaction id in which this buffer has been cowed.

In the header part of file journal-head.h, it defines
   typedef unsigned int  tid_t;
We should use type tid_t to define transaction id fields.

Change the field "b_cow_tid" of struct journal_head from
type unsigned to tid_t.
Signed-off-by: NWang Sheng-Hui <shhuiw@gmail.com>
Acked-by: NAmir Goldstein <amir73il@users.sf.net>
Signed-off-by: NJan Kara <jack@suse.cz>

include/linux/journal-head.h

@@ -45,7 +45,7 @@ struct journal_head {
 	 * has been cowed
 	 * [jbd_lock_bh_state()]
 	 */
-	unsigned b_cow_tid;
+	tid_t b_cow_tid;
 
 	/*
 	 * Copy of the buffer data frozen for writing to the log.