ovpn: use correct array size to parse nested attributes in ovpn_nl_key_swap_doit

ovpn: use correct array size to parse nested attributes in ovpn_nl_key_swap_doit

In ovpn_nl_key_swap_doit, the attributes array used to parse the
OVPN_A_KEYCONF uses OVPN_A_PEER_MAX instead of
OVPN_A_KEYCONF_MAX. Note that this does not cause any bug, since
currently OVPN_A_KEYCONF_MAX < OVPN_A_PEER_MAX.

The wrong constant was introduced by commit 203e2bf55990
("ovpn: implement key add/get/del/swap via netlink")

Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Antonio Quartulli <antonio@openvpn.net>