[TCP] tcp-lp: prevent chance for oops

This patch fix the chance for tcp_lp_remote_hz_estimator return 0, if
0 < rhz < 64. It also make sure the flag LP_VALID_RHZ is set
correctly.

Signed-off-by: Wong Hoi Sing Edison <hswong3i@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>