Resolve cache leak from hard link

Add the 'ihold()' into 'simplefs_link' function

The hard link cache and old file cache are both in the cache list,
the "kill_block_super" attempts to free the inode cache, it will
decrement "i_count" twice. Therefore, if 'i_count' is not 0,
'destroy_inode' will not be called.

Author: RoyWFHuang

inode.c

@@ -897,6 +897,7 @@ static int simplefs_link(struct dentry *old_dentry,
     brelse(bh);
 
     inode_inc_link_count(inode);
+    ihold(inode);
     d_instantiate(dentry, inode);
     return ret;