net/ipv4/fib_semantics.c uses a hash table (fib_info_laddrhash)
in which fib_sync_down_addr() can locate fib_info
based on IPv4 local address.

This hash table is resized based on total number of
hashed fib_info, but the hash function is only
using the local address.

For hosts having many active network namespaces,
all fib_info for loopback devices (IPv4 address 127.0.0.1)
are hashed into a single bucket, making netns dismantles
very slow.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Reviewed-by: NDavid Ahern <dsahern@kernel.org>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>