jbd2: add the b_cow_tid field to journal_head struct

The b_cow_tid field will be used by the ext4 snapshots code to store
the transaction id when the buffer was last cowed.

Merging this patch to mainline will allow users to test ext4 snapshots
as a standalone module, without the need to patch and install a
development kernel.

On 64bit machines this field uses fills in a padding "hole" and does
not increase the size of the struct.  On a 32bit machine this patch
increases the size of the struct from 60 to 64 bytes.

Signed-off-by: Amir Goldstein <amir73il@users.sf.net>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>