gfs2: Clean up on-stack transactions

Replace the TR_ALLOCED flag by its inverse, TR_ONSTACK: that way, the flag only
needs to be set in the exceptional case of on-stack transactions.  Split off
__gfs2_trans_begin from gfs2_trans_begin and use it to replace the open-coded
version in gfs2_ail_empty_gl.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
diff --git a/fs/gfs2/incore.h b/fs/gfs2/incore.h
index 8e1ab8e..c3f6dd3 100644
--- a/fs/gfs2/incore.h
+++ b/fs/gfs2/incore.h
@@ -490,7 +490,7 @@ struct gfs2_quota_data {
 enum {
 	TR_TOUCHED = 1,
 	TR_ATTACHED = 2,
-	TR_ALLOCED = 3,
+	TR_ONSTACK = 3,
 };
 
 struct gfs2_trans {