ipv4: remove fib_devindex_hashfn()

ipv4: remove fib_devindex_hashfn()

fib_devindex_hashfn() converts a 32bit ifindex value to a 8bit hash.

It makes no sense doing this from fib_info_hashfn() and
fib_find_info_nh().

It is better to keep as many bits as possible to let
fib_info_hashfn_result() have better spread.

Only fib_info_devhash_bucket() needs to make this operation,
we can 'inline' trivial fib_devindex_hashfn() in it.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20241004134720.579244-2-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>