Slightly reduce BufMgrLock contention

This patch prevents btbulkdelete() from calling WriteNoReleaseBuffer()
several times for the same buffer.  Thus it saves a few
LWLockAquire(BufMgrLock, LW_EXCLUSIVE) and LWLockRelease(BufMgrLock)
calls.

Maybe we do not need a BufMgrLock at all, because we have a super
exclusive lock on the buffer?  I was not sure and decided to stay on
the safe side ...

Servus
 Manfred
diff -ruN ../base/src/backend/access/nbtree/nbtree.c src/backend/access/nbtree/nbtree.c
--- ../base/src/backend/access/nbtree/nbtree.c    2002-06-21 02:12:14.000000000 +0200
+++ src/backend/access/nbtree/nbtree.c    2002-08-30 11:44:35.000000000 +0200
@@ -615,6 +615,7 @@
     {
         Buffer        buf;
         BlockNumber lockedBlock = InvalidBlockNumber;
+        bool        dirty = false;

         /* we have the buffer pinned and locked */
         buf = so->btso_curbuf;
@@ -662,14 +663,19 @@
                     LockBuffer(buf, BUFFER_LOCK_UNLOCK);
                     LockBufferForCleanup(buf);
                     lockedBlock = blkno;
+                    dirty = false;
                 }
                 else
                 {
                     /* Okay to delete the item from the page */
                     _bt_itemdel(rel, buf, current);

-                    /* Mark buffer dirty, but keep the lock and pin */
-                    WriteNoReleaseBuffer(buf);
+                    if (!dirty)
+                    {
+                        /* Mark buffer dirty, but keep the lock and pin */
+                        WriteNoReleaseBuffer(buf);
+                        dirty = true;
+                    }

                     tuples_removed += 1;
                 }