dcache: d_splice_alias should ignore DCACHE_DISCONNECTED

Any IS_ROOT() alias should be safe to use; there's nothing special about
DCACHE_DISCONNECTED dentries.

Note that this is in fact useful for filesystems such as btrfs which can
legimately encounter a directory with a preexisting IS_ROOT alias on a
lookup that crosses into a subvolume.  (Those aliases are currently
marked DCACHE_DISCONNECTED--but not really for any good reason, and
we'll change that soon.)
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/dcache.c

@@ -2649,9 +2649,9 @@ static void __d_materialise_dentry(struct dentry *dentry, struct dentry *anon)
  * @inode:  the inode which may have a disconnected dentry
  * @dentry: a negative dentry which we want to point to the inode.
  *
- * If inode is a directory and has a 'disconnected' dentry (i.e. IS_ROOT and
- * DCACHE_DISCONNECTED), then d_move that in place of the given dentry
- * and return it, else simply d_add the inode to the dentry and return NULL.
+ * If inode is a directory and has an IS_ROOT alias, then d_move that in
+ * place of the given dentry and return it, else simply d_add the inode
+ * to the dentry and return NULL.
  *
  * If a non-IS_ROOT directory is found, the filesystem is corrupt, and
  * we should error out: directories can't have multiple aliases.
@@ -2678,7 +2678,7 @@ struct dentry *d_splice_alias(struct inode *inode, struct dentry *dentry)
 		spin_lock(&inode->i_lock);
 		new = __d_find_any_alias(inode);
 		if (new) {
-			if (!IS_ROOT(new) || !(new->d_flags & DCACHE_DISCONNECTED)) {
+			if (!IS_ROOT(new)) {
 				spin_unlock(&inode->i_lock);
 				dput(new);
 				return ERR_PTR(-EIO);