Split nhop_ref_object().

Now nhop_ref_object() unconditionally acquires a reference, and the new
nhop_try_ref_object() uses refcount_acquire_if_not_zero() to
conditionally acquire a reference. Since the former is cheaper, use it
when we know that the initial counter value is non-zero. No functional
change intended.

Reviewed by: melifaro
Differential Revision: https://reviews.freebsd.org/D25535