Turn pppoe(4) back to kernel lock. We can't predict netlock state within

Turn pppoe(4) back to kernel lock. We can't predict netlock state within
pppoe_start(), so we can't use it for pppoe(4) data protection. Except
input path, pppoe(4) always accessed with kernel lock held, so grab it
around pppoeintr() too.

Interfaces should not use netlock for their data protection. They should
rely on kernel lock or implement their own.

ok bluhm@ bket@