tcp: tcp_gro_dev_warn() cleanup

Use DO_ONCE_LITE_IF() and __cold attribute to put tcp_gro_dev_warn()
out of line.

This also allows the message to be printed again after a
"echo 1 > /sys/kernel/debug/clear_warn_once"

Also add a READ_ONCE() when reading device mtu, as it could
be changed concurrently.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20231130184135.4130860-1-edumazet@google.com


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