gfs2: use is_subdir()

... instead of reimplementing it with misguiding name (is_ancestor(x, y)
would normally imply "x is an ancestor of y", not the other way round).
With races, while we are at it...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>