net: Pass 'net' struct as first argument to fib6_info_hw_flags_set()

The next patch will emit notification when hardware flags are changed,
in case that fib_notify_on_flag_change sysctl is set to 1.

To know sysctl values, net struct is needed.
This change is consistent with the IPv4 version, which gets 'net' struct
as its first argument.

Currently, the only callers of this function are mlxsw and netdevsim.
Patch the callers to pass net.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>