protect ip->valid and ip->nlink with sleep lock in iput()

file.h

@@ -14,8 +14,8 @@ struct inode {
   uint dev;           // Device number
   uint inum;          // Inode number
   int ref;            // Reference count
-  struct sleeplock lock;
-  int valid;          // remainder has been read from disk?
+  struct sleeplock lock; // protects everything below here
+  int valid;          // inode has been read from disk?
 
   short type;         // copy of disk inode
   short major;