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: Wang Sheng-Hui <shhuiw@gmail.com>
Acked-by: Amir Goldstein <amir73il@users.sf.net>
Signed-off-by: Jan Kara <jack@suse.cz>