routing: restore opt_route.h in route.c, fib_algo teardown was compiled out

Removal of opt_route.h left route.c without FIB_ALGO, skipping
fib_destroy_rib() in rt_table_destroy(). Explicitly guard it to clean up
fib_algo instances and callouts before releasing the rib_lock rmlock,
preventing a page fault in softclock and VNET resource leaks.

Approved by:	pouria
Fixes:		254b23eb1f ("routing: Retire ROUTE_MPATH compile option")
Sponsored by:	Netflix
Differential Revision:	https://reviews.freebsd.org/D60158

sys/net/route.c

@@ -36,6 +36,7 @@
 #include "opt_inet.h"
 #include "opt_inet6.h"
 #include "opt_mrouting.h"
+#include "opt_route.h"
 
 #include <sys/param.h>
 #include <sys/systm.h>