i40e: fix the panic when running bpf in xdpdrv mode

Fix this panic by adding more rules to calculate the value of @rss_size_max
which could be used in allocating the queues when bpf is loaded, which,
however, could cause the failure and then trigger the NULL pointer of
vsi->rx_rings. Prio to this fix, the machine doesn't care about how many
cpus are online and then allocates 256 queues on the machine with 32 cpus
online actually.

Once the load of bpf begins, the log will go like this "failed to get
tracking for 256 queues for VSI 0 err -12" and this "setup of MAIN VSI
failed".

Thus, I attach the key information of the crash-log here.

BUG: unable to handle kernel NULL pointer dereference at
0000000000000000
RIP: 0010:i40e_xdp+0xdd/0x1b0 [i40e]
Call Trace:
[2160294.717292]  ? i40e_reconfig_rss_queues+0x170/0x170 [i40e]
[2160294.717666]  dev_xdp_install+0x4f/0x70
[2160294.718036]  dev_change_xdp_fd+0x11f/0x230
[2160294.718380]  ? dev_disable_lro+0xe0/0xe0
[2160294.718705]  do_setlink+0xac7/0xe70
[2160294.719035]  ? __nla_parse+0xed/0x120
[2160294.719365]  rtnl_newlink+0x73b/0x860

Fixes: 41c445f ("i40e: main driver core")
Co-developed-by: Shujin Li <lishujin@kuaishou.com>
Signed-off-by: Shujin Li <lishujin@kuaishou.com>
Signed-off-by: Jason Xing <xingwanli@kuaishou.com>
Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/intel/i40e/i40e_main.c

@@ -12357,6 +12357,7 @@ static int i40e_sw_init(struct i40e_pf *pf)
 {
 	int err = 0;
 	int size;
+	u16 pow;
 
 	/* Set default capability flags */
 	pf->flags = I40E_FLAG_RX_CSUM_ENABLED |
@@ -12375,6 +12376,11 @@ static int i40e_sw_init(struct i40e_pf *pf)
 	pf->rss_table_size = pf->hw.func_caps.rss_table_size;
 	pf->rss_size_max = min_t(int, pf->rss_size_max,
 				 pf->hw.func_caps.num_tx_qp);
+
+	/* find the next higher power-of-2 of num cpus */
+	pow = roundup_pow_of_two(num_online_cpus());
+	pf->rss_size_max = min_t(int, pf->rss_size_max, pow);
+
 	if (pf->hw.func_caps.rss) {
 		pf->flags |= I40E_FLAG_RSS_ENABLED;
 		pf->alloc_rss_size = min_t(int, pf->rss_size_max,