get rid of autofs_getpath()

allow wq->name.name to point not at the beginning of the object containing
the string, with wq->offset telling how far into it we are.  Then we can
bloody well just use dentry_path_raw() instead of autofs_getpath() -
the only real difference is that dentry_path_raw() puts the result into
the end of buffer and returns where it starts.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>