[GFS2] Add an additional argument to gfs2_trans_add_bh()

This adds an extra argument to gfs2_trans_add_bh() to indicate whether the
bh being added to the transaction is metadata or data. Its currently unused
since all existing callers set it to 1 (metadata) but following patches will
make use of it.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>