iflib: Fix compiler warnings

Some of the QUAD sysctls are actually for unsigned quad values.
Switch to using UQUAD instead, as that is meant for unsigned.

Reviewed by: erj, jhb
Obtained from: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D44620

(cherry picked from commit 303dea74c2cb3a41fba455fce8577993e637c3da)