Fix spinlock usage in UnpinBuffer()

[Qingqing Zhou <zhouqq(at)cs(dot)toronto(dot)edu>]

There is no LWLock protecting the spinlock in UnpinBuffer(), so we need do
so ourselves. I also checked other NoHoldOff spinlock, seems they are ok.

Regards,
Qingqing

Index: bufmgr.c
===================================================================
RCS file: /projects/cvsroot/pgsql/src/backend/storage/buffer/bufmgr.c,v
retrieving revision 1.200
diff -c -r1.200 bufmgr.c
*** bufmgr.c	22 Nov 2005 18:17:19 -0000	1.200
--- bufmgr.c	28 Dec 2005 09:33:18 -0000
***************
*** 812,817 ****
--- 812,818 ----
  		Assert(!LWLockHeldByMe(buf->io_in_progress_lock));

  		/* NoHoldoff ensures we don't lose control before sending signal */
+ 		HOLD_INTERRUPTS();
  		LockBufHdr_NoHoldoff(buf);

  		/* Decrement the shared reference count */
***************
*** 847,852 ****
--- 848,856 ----
  		else
  			UnlockBufHdr_NoHoldoff(buf);

+ 		/* safe to accept interrupts now */
+ 		RESUME_INTERRUPTS();
+
  		/*
  		 * If VACUUM is releasing an otherwise-unused buffer, send it to the
  		 * freelist for near-term reuse.  We put it at the tail so that it