fib_rules: move common handling of newrule delrule msgs into fib_nl2rule

This reduces code duplication in the fib rule add and del paths.
Get rid of validate_rulemsg. This became obvious when adding duplicate
extack support in fib newrule/delrule error paths.

Signed-off-by: Roopa Prabhu <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

Author: roopa-prabhu