namei: fix logic and infinite loop of symlinks

namei: fix logic and infinite loop of symlinks

Don't follow the path if a component is not directory.  It doesn't make sense
to support something like:

	$ touch a b
	$ namei a/b  <-- where "a" is not directory

The support for infinite loop of symbolic links is a strange wish only. The
stack size is very limited. Try:

	$ ln -s x
	$ namei x/x

[Migration note: severity="low"]

Signed-off-by: Karel Zak <kzak@redhat.com>