tools/fib_multibind: Remove a redundant calculation

No functional change intended.

Reported by: kevans
Fixes: 3fa552149885 ("tools: Add a small program to demonstrate FIB handling in bind(2)")
Sponsored by: Klara, Inc.
Sponsored by: Stormshield

(cherry picked from commit d46d45bf4b196cd2daba42d5413f8e4d7ffedeed)