ipv4: avoid quadratic behavior in FIB insertion of common address

Mix netns into all IPv4 FIB hashes to avoid massive collision when
inserting the same address in many netns.

Signed-off-by: Alexandre Ferrieux <alexandre.ferrieux@orange.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20241001231438.3855035-1-alexandre.ferrieux@orange.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>