fib_trie: Move fib_find_alias to file where it is used

The function fib_find_alias is only accessed by functions in fib_trie.c as
such it makes sense to relocate it and cast it as static so that the
compiler can take advantage of optimizations it can do to it as a local
function.

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