GFS2: Introduce new gfs2_log_header_v2

This patch adds a new structure called gfs2_log_header_v2 which is used
to store expanded fields into previously unused areas of the log headers
(i.e., this change is backwards compatible).  Some of these are used for
debug purposes so we can backtrack when problems occur.  Others are
reserved for future expansion.

This patch is based on a prototype from Steve Whitehouse.

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