vfs, seqfile: make mangle_path() global

Impact: expose new VFS API

make mangle_path() available, as per the suggestions of Christoph Hellwig
and Al Viro:

  http://lkml.org/lkml/2008/11/4/338



Signed-off-by: Török Edwin <edwintorok@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>