routing: Use a better error number in sysctl_fibs()

ENOTCAPABLE is for capsicum and its use here is inappropriate. In
particular, note that syscallret() treats this value specially.

Reviewed by: glebius, pouria, zlei
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D56481

(cherry picked from commit 8de0fc10a1c2d65bdb39eff862266ab1f87902c8)