Move fib_rte_to_nh_flags() from net/route_var.h to net/route/nhop_ctl.c.

No functional changes.
Initially this function was created to perform runtime flag conversions
for the previous incarnation of fib lookup functions. As these functions
got deprecated, move the function to the file with the only remaining
caller. Lastly, rename it to convert_rt_to_nh_flags() to follow the
naming notation.