inetpeer: do not get a refcount in inet_getpeer()

inetpeer: do not get a refcount in inet_getpeer()

All inet_getpeer() callers except ip4_frag_init() don't need
to acquire a permanent refcount on the inetpeer.

They can switch to full RCU protection.

Move the refcount_inc_not_zero() into ip4_frag_init(),
so that all the other callers no longer have to
perform a pair of expensive atomic operations on
a possibly contended cache line.

inet_putpeer() no longer needs to be exported.

After this patch, my DUT can receive 8,400,000 UDP packets
per second targeting closed ports, using 50% less cpu cycles
than before.

Also change two calls to l3mdev_master_ifindex() by
l3mdev_master_ifindex_rcu() (Ido ideas)

Fixes: 8c2bd38b95f7 ("icmp: change the order of rate limits")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20241215175629.1248773-5-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>