netlink: fix if_allocdescr() malloc options argument.

The current value was based on the initial revision of D37566 and
was not updated afterwards.

MFC after: 1 day
Approved by: re(cperciva)

(cherry picked from commit 6694cb1f5784d6ceacd684d29b3e3012e1e4e4d5)
(cherry picked from commit 8c60d469d3faa98f6b0e49173f16a8603c4c4654)