fib_trie: Minor cleanups to fib_table_flush_external

This change just does a couple of minor cleanups on
fib_table_flush_external.  Specifically it addresses the fact that resize
was being called even though nothing was being removed from the table, and
it drops an unecessary indent since we could just call continue on the
inverse of the fi && flag check.

Signed-off-by: Alexander Duyck <alexander.h.duyck@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>