JFS: Fix structure alignment problem on 64-bit machines

The code in xtLog assumes that the structures xdlistlock_t and
maplock_t are the same size, as they are overlaid.  On 64-bit
machines, this was not the case.

Ensure that the xdlistlock_t is 16 bytes in length regardless of
the size of a pointer.

fs/jfs/jfs_txnmgr.h

@@ -251,10 +251,17 @@ typedef struct {
 	u8 count;		/* 1: number of pxd/xad */
 				/* (8) */
 
-	void *xdlist;		/* 4: pxd/xad list */
-	s32 rsrvd;		/* 4: */
+	/*
+	 * We need xdlistlock_t to be 64 bits (8 bytes), regardless of
+	 * whether void * is 32 or 64 bits
+	 */
+	union {
+		void *_xdlist;	/* pxd/xad list */
+		s64 pad;	/* 8: Force 64-bit xdlist size */
+	} union64;
 } xdlistlock_t;			/* (16): */
 
+#define xdlist union64._xdlist
 
 /*
  *	commit