xfs: introduce protection for drop nlink

When abnormal drop_nlink are detected on the inode,
return error, to avoid corruption propagation.
Signed-off-by: Cheng Lin <cheng.lin130@zte.com.cn>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>

fs/xfs/xfs_inode.c

@@ -918,6 +918,13 @@ xfs_droplink(
 	xfs_trans_t *tp,
 	xfs_inode_t *ip)
 {
+	if (VFS_I(ip)->i_nlink == 0) {
+		xfs_alert(ip->i_mount,
+			  "%s: Attempt to drop inode (%llu) with nlink zero.",
+			  __func__, ip->i_ino);
+		return -EFSCORRUPTED;
+	}
+
 	xfs_trans_ichgtime(tp, ip, XFS_ICHGTIME_CHG);
 
 	drop_nlink(VFS_I(ip));