gfs2: changes to gfs2_log_XXX_bio

Change gfs2_log_XXX_bio family of functions so they can be used
with different bios, not just sdp->sd_log_bio.

This patch also contains some clean up suggested by Andreas.

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