nexthop: Fix type of event_type in call_nexthop_notifiers

Clang warns:

net/ipv4/nexthop.c:841:30: warning: implicit conversion from enumeration
type 'enum nexthop_event_type' to different enumeration type 'enum
fib_event_type' [-Wenum-conversion]
        call_nexthop_notifiers(net, NEXTHOP_EVENT_DEL, nh);
        ~~~~~~~~~~~~~~~~~~~~~~      ^~~~~~~~~~~~~~~~~
1 warning generated.

Use the right type for event_type so that clang does not warn.

Fixes: 8590ceed ("nexthop: add support for notifiers")
Link: https://github.com/ClangBuiltLinux/linux/issues/1038


Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>