net/ipv6: fix bool/int mismatch for skip_notify_on_dev_down

skip_notify_on_dev_down ctl table expects this field
to be an int (4 bytes), not a bool (1 byte).

Because proc_dou8vec_minmax() was added in 5.13,
this patch converts skip_notify_on_dev_down to an int.

Following patch then converts the field to u8 and use proc_dou8vec_minmax().

Fixes: 7c6bb7d2

 ("net/ipv6: Add knob to skip DELROUTE message on device down")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Acked-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>