ipvs: check that ip_vs_conn_tab_bits is between 8 and 20

ip_vs_conn_tab_bits may be provided by the user through the
conn_tab_bits module parameter. If this value is greater than 31, or
less than 0, the shift operator used to derive tab_size causes undefined
behaviour.

Fix this checking ip_vs_conn_tab_bits value to be in the range specified
in ipvs Kconfig. If not, simply use default value.

Fixes: 6f7edb4881bf ("IPVS: Allow boot time change of hash size")
Reported-by: Yi Chen <[e-mail]>
Signed-off-by: Andrea Claudi <[e-mail]>
Acked-by: Julian Anastasov <[e-mail]>
Acked-by: Simon Horman <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

net/netfilter/ipvs/ip_vs_conn.c

···
 	int idx;
 
 	/* Compute size and mask */
+	if (ip_vs_conn_tab_bits < 8 || ip_vs_conn_tab_bits > 20) {
+		pr_info("conn_tab_bits not in [8, 20]. Using default value\n");
+		ip_vs_conn_tab_bits = CONFIG_IP_VS_TAB_BITS;
+	}
 	ip_vs_conn_tab_size = 1 << ip_vs_conn_tab_bits;
 	ip_vs_conn_tab_mask = ip_vs_conn_tab_size - 1;