The call to namei("/dir/file/") currently fails with ENOENT when
"/dir/file" exists. A more standard way is to return ENOTDIR
instead. This way calls to stat, open, rename, etc. will be in
line with the POSIX spec.

It is also useful to rename() implementation which needs to
differentiate behaviour between the case in which target does not
exist and the case in which it does but the path has trailing slash
and the last component is not a directory.

In addition to that the check was performed in an inconsistent matter
- only when dentry lookup failed. This change makes the check
performed always.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>