cephfs: potential adjust failure in lru_expire

Fix: the first adjust is no needed,it will never take real effect.
     the second 'adjust' may never get the chance to be executed
     suppose we can reach the second 'adjust', it will crash because the bottom list is empty now.

Signed-off-by: dongdong tao <tdd21151186@gmail.com>

src/include/lru.h

@@ -157,14 +157,14 @@ class LRU {
 
   // expire -- expire a single item
   LRUObject *lru_get_next_expire() {
+    adjust();
     // look through tail of bot
     while (bottom.size()) {
       LRUObject *p = bottom.back();
       if (!p->lru_pinned) return p;
 
       // move to pintail
       pintail.push_front(&p->lru_link);
-      adjust();
     }
 
     // ok, try head then
@@ -174,7 +174,6 @@ class LRU {
 
       // move to pintail
       pintail.push_front(&p->lru_link);
-      adjust();
     }
 
     // no luck!