Refactor fib4/fib6 functions.

No functional changes.

• Make lookup path of fib<4|6>_lookup_debugnet() separate functions (fib<46>_lookup_rt()). These will be used in the control plane code requiring unlocked radix operations and actual prefix pointer.
• Make lookup part of fib<4|6>_check_urpf() separate functions. This change simplifies the switch to alternative lookup implementations, which helps algorithmic lookups introduction.
• While here, use static initializers for IPv4/IPv6 keys

Differential Revision: https://reviews.freebsd.org/D27405