nullfs: Use an a_gen field to cast to vop_generic_args

Instead of casting a vop_F_args object to vop_generic_args, use a
vop_F_args.a_gen field when calling null_bypass(). This way we don't
hardcode the vop_generic_args data type in the callers of null_bypass().

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
and easier to maintain.

Pointed out by: jrtc27
Reviewed by: kib, oshogbo
Accepted by: oshogbo (mentor)
Differential Revision: https://reviews.freebsd.org/D37359