ovl: ovl_obtain_alias(): don't call d_instantiate_anon() for old

The situation is the same as for __d_obtain_alias() (which is what that
thing is parallel to) - if we find a preexisting alias, we want to grab it,
drop the inode and return the alias we'd found.

The only thing d_instantiate_anon() does compared to that is spurious
security_d_instiate() that has already been done to that dentry with exact
same arguments.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>