Fix mountpoint reference leakage in linkat

Recent changes to retry on ESTALE in linkat
(commit 442e31ca)
introduced a mountpoint reference leak and a small memory
leak in case a filesystem link operation returns ESTALE
which is pretty normal for distributed filesystems like
lustre, nfs and so on.
Free old_path in such a case.

[AV: there was another missing path_put() nearby - on the previous
goto retry]

Signed-off-by: Oleg Drokin: <green@linuxhacker.ru>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>