Add new options pppoe-padi-timeout and pppoe-padi-attempts.
These modifications are the similar to the ones done on
pppoe-discovery in commit 70a8ad3d ("pppoe-discovery: add options to
tune discovery timeout and attempts", 2017-12-07).
Signed-off-by: Alexis Cellier <alexis.cellier@smile.fr> Signed-off-by: Paul Mackerras <paulus@ozlabs.org>