Passing NULL ns to label print fns is used as shorthand in the a few
places to mean default to the current ns (ie. aa_current_ns()). However
not all the print routines support this.

Instead of fixing the call sites that pass null into the label_print
fns, update all the label_print fns to support this use.
Signed-off-by: John Johansen <john.johansen@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>