Fix Symbolic link remove problem

    When we create symbolic link, in simplefs_new_inode function, the symbolic
does not use ei_block to record the using block. But in simplefs_unlink function
we use the ei_block to clean the bitmap, this makes the unlink will clean the "0"
bit in bitmap.

Close #58

inode.c

@@ -658,7 +658,8 @@ static int simplefs_unlink(struct inode *dir, struct dentry *dentry)
     mark_inode_dirty(inode);
 
     /* Free inode and index block from bitmap */
-    put_blocks(sbi, bno, 1);
+    if (S_ISLNK(inode->i_mode))
+        put_blocks(sbi, bno, 1);
     put_inode(sbi, ino);
 
     return ret;