net/ipv6: Fix misuse of proc_dointvec "flowlabel_reflect"

/proc/sys/net/ipv6/flowlabel_reflect assumes written value to be in the
range of 0 to 3. Use proc_dointvec_minmax instead of proc_dointvec.

Fixes: 323a53c4 ("ipv6: tcp: enable flowlabel reflection in some RST packets")
Signed-off-by: Eiichi Tsukata <devel@etsukata.com>
Signed-off-by: David S. Miller <davem@davemloft.net>