vfs: fix d_ancestor() case in d_materialize_unique

In d_materialise_unique() there are 3 subcases to the 'aliased dentry'
case; in two subcases the inode i_lock is properly released but this
does not occur in the -ELOOP subcase.

This seems to have been introduced by commit 1836750115 ("fix loop
checks in d_materialise_unique()").

Signed-off-by: Michel Lespinasse <walken@google.com>
Cc: stable@vger.kernel.org # v3.0+
[ Added a comment, and moved the unlock to where we generate the -ELOOP,
  which seems to be more natural.

  You probably can't actually trigger this without a buggy network file
  server - d_materialize_unique() is for finding aliases on non-local
  filesystems, and the d_ancestor() case is for a hardlinked directory
  loop.

  But we should be robust in the case of such buggy servers anyway. ]
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/dcache.c

@@ -2404,6 +2404,7 @@ struct dentry *d_materialise_unique(struct dentry *dentry, struct inode *inode)
 			if (d_ancestor(alias, dentry)) {
 				/* Check for loops */
 				actual = ERR_PTR(-ELOOP);
+				spin_unlock(&inode->i_lock);
 			} else if (IS_ROOT(alias)) {
 				/* Is this an anonymous mountpoint that we
 				 * could splice into our tree? */
@@ -2413,7 +2414,7 @@ struct dentry *d_materialise_unique(struct dentry *dentry, struct inode *inode)
 				goto found;
 			} else {
 				/* Nope, but we must(!) avoid directory
-				 * aliasing */
+				 * aliasing. This drops inode->i_lock */
 				actual = __d_unalias(inode, dentry, alias);
 			}
 			write_sequnlock(&rename_lock);