vfs, seqfile: export mangle_path() generally

mangle_path() is trivial enough to make  export restrictions on it
pointless - so change the export from EXPORT_SYMBOL_GPL to EXPORT_SYMBOL.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Acked-by: Al Viro <viro@ZenIV.linux.org.uk>