gfs2: Remove ordered write mode handling from gfs2_trans_add_data

In journaled data mode, we need to add each buffer head to the current
transaction.  In ordered write mode, we only need to add the inode to
the ordered inode list.  So far, both cases are handled in
gfs2_trans_add_data.  This makes the code look misleading and is
inefficient for small block sizes as well.  Handle both cases separately
instead.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>