kernel: hfs: add sanity check for file name length

[Eugene Teo <eugene () redhat com>]

https://lkml.org/lkml/2011/11/9/303
https://bugzilla.redhat.com/show_bug.cgi?id=755431
http://git.kernel.org/linus/bc5b8a9003132ae44559edd63a1623

On a corrupted file system the ->len field could be wrong leading to a
buffer overflow.

Thanks, Eugene