smack: make mac_smack_fix() deal somewhat sensible with non-absolute paths

smack: make mac_smack_fix() deal somewhat sensible with non-absolute paths

This tries to improve the mac_smack_fix() logic a bit, by properly
handling non-absolute paths.

It's still pretty broken though, which is sad for security technology:
non-normalized paths (for example "/usr/../dev/sda") will still not be
treated correctly. I am not sure how to fix that properly though, and I
don't understand SMACK well enough to do so. This fix hence just fixes
to most obvious glaring issue.