fix inode leaks on d_splice_alias() failure exits

d_splice_alias() callers expect it to either stash the inode reference
into a new alias, or drop the inode reference.  That makes it possible
to just return d_splice_alias() result from ->lookup() instance, without
any extra housekeeping required.

Unfortunately, that should include the failure exits.  If d_splice_alias()
returns an error, it leaves the dentry it has been given negative and
thus it *must* drop the inode reference.  Easily fixed, but it goes way
back and will need backporting.

Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/dcache.c

@@ -2673,11 +2673,13 @@ struct dentry *d_splice_alias(struct inode *inode, struct dentry *dentry)
 			if (!IS_ROOT(new)) {
 				spin_unlock(&inode->i_lock);
 				dput(new);
+				iput(inode);
 				return ERR_PTR(-EIO);
 			}
 			if (d_ancestor(new, dentry)) {
 				spin_unlock(&inode->i_lock);
 				dput(new);
+				iput(inode);
 				return ERR_PTR(-EIO);
 			}
 			write_seqlock(&rename_lock);