Instead of using a vop_F_args.a_gen field, always cast a vop_F_args
object to vop_generic_args.
Before this change, there were 3 null_bypass() calls using
a vop_F_args.a_gen field and 5 null_bypass() calls using a cast to
vop_generic_args. This change makes all null_bypass() calls consistent.
Pointed out by: jrtc27