[GFS2] Use BUG_ON() rather then if (...) BUG();

This issue was raised by:
Eric Sesterhenn <snakebyte@gmx.de>

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>
diff --git a/fs/gfs2/lm.c b/fs/gfs2/lm.c
index cc74422..f86f11f 100644
--- a/fs/gfs2/lm.c
+++ b/fs/gfs2/lm.c
@@ -101,8 +101,8 @@
 	va_end(args);
 
 	fs_err(sdp, "about to withdraw from the cluster\n");
-	if (sdp->sd_args.ar_debug)
-		BUG();
+	BUG_ON(sdp->sd_args.ar_debug);
+	
 
 	fs_err(sdp, "waiting for outstanding I/O\n");