net/sched: cls_flower: use nla_get_be32 for TCA_FLOWER_KEY_FLAGS

The existing code is functionally correct: iproute2 parses the ip_flags
argument for tc-flower and really packs it as big endian into the
TCA_FLOWER_KEY_FLAGS netlink attribute. But there is a problem in the
fact that W=1 builds complain:

net/sched/cls_flower.c:1047:15: warning: cast to restricted __be32

This is because we should use the dedicated helper for obtaining a
__be32 pointer to the netlink attribute, not a u32 one. This ensures
type correctness for be32_to_cpu.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>