xfs: fix broken i_nlink accounting for whiteout tmpfile inode

XFS uses the internal tmpfile() infrastructure for the whiteout inode
used for RENAME_WHITEOUT operations. For tmpfile inodes, XFS allocates
the inode, drops di_nlink, adds the inode to the agi unlinked list,
calls d_tmpfile() which correspondingly drops i_nlink of the vfs inode,
and then finishes the common inode setup (e.g., clear I_NEW and unlock).

The d_tmpfile() call was originally made inxfs_create_tmpfile(), but was
pulled up out of that function as part of the following commit to
resolve a deadlock issue:

	330033d xfs: fix tmpfile/selinux deadlock and initialize security

As a result, callers of xfs_create_tmpfile() are responsible for either
calling d_tmpfile() or fixing up i_nlink appropriately. The whiteout
tmpfile allocation helper does neither. As a result, the vfs ->i_nlink
becomes inconsistent with the on-disk ->di_nlink once xfs_rename() links
it back into the source dentry and calls xfs_bumplink().

Update the assert in xfs_rename() to help detect this problem in the
future and update xfs_rename_alloc_whiteout() to decrement the link
count as part of the manual tmpfile inode setup.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

fs/xfs/xfs_inode.c

@@ -2879,7 +2879,13 @@ xfs_rename_alloc_whiteout(
 	if (error)
 		return error;
 
-	/* Satisfy xfs_bumplink that this is a real tmpfile */
+	/*
+	 * Prepare the tmpfile inode as if it were created through the VFS.
+	 * Otherwise, the link increment paths will complain about nlink 0->1.
+	 * Drop the link count as done by d_tmpfile(), complete the inode setup
+	 * and flag it as linkable.
+	 */
+	drop_nlink(VFS_I(tmpfile));
 	xfs_finish_inode_setup(tmpfile);
 	VFS_I(tmpfile)->i_state |= I_LINKABLE;
 
@@ -3147,7 +3153,7 @@ xfs_rename(
 	 * intermediate state on disk.
 	 */
 	if (wip) {
-		ASSERT(wip->i_d.di_nlink == 0);
+		ASSERT(VFS_I(wip)->i_nlink == 0 && wip->i_d.di_nlink == 0);
 		error = xfs_bumplink(tp, wip);
 		if (error)
 			goto out_trans_abort;