idpf: fix null-ptr-deref in idpf_features_check

idpf_features_check is used to validate the TX packet. skb header
length is compared with the hardware supported value received from
the device control plane. The value is stored in the adapter structure
and to access it, vport pointer is used. During reset all the vports
are released and the vport pointer that the netdev private structure
points to is NULL.

To avoid null-ptr-deref, store the max header length value in netdev
private structure. This also helps to cache the value and avoid
accessing adapter pointer in hot path.

BUG: kernel NULL pointer dereference, address: 0000000000000068
...
RIP: 0010:idpf_features_check+0x6d/0xe0 [idpf]
Call Trace:
 <TASK>
 ? __die+0x23/0x70
 ? page_fault_oops+0x154/0x520
 ? exc_page_fault+0x76/0x190
 ? asm_exc_page_fault+0x26/0x30
 ? idpf_features_check+0x6d/0xe0 [idpf]
 netif_skb_features+0x88/0x310
 validate_xmit_skb+0x2a/0x2b0
 validate_xmit_skb_list+0x4c/0x70
 sch_direct_xmit+0x19d/0x3a0
 __dev_queue_xmit+0xb74/0xe70
 ...

Fixes: a251eee6 ("idpf: add SRIOV support and other ndo_ops")
Reviewed-by: Madhu Chititm <madhu.chittim@intel.com>
Signed-off-by: Pavan Kumar Linga <pavan.kumar.linga@intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Tested-by: Samuel Salin <Samuel.salin@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>