After PF reset and ethtool -t there was call trace in dmesg
sometimes leading to panic. When there was some time, around 5
seconds, between reset and test there were no errors.

Problem was that pf reset calls i40e_vsi_close in prep_for_reset
and ethtool -t calls i40e_vsi_close in diag_test. If there was not
enough time between those commands the second i40e_vsi_close starts
before previous i40e_vsi_close was done which leads to crash.

Add check to diag_test if pf is in reset and don't start offline
tests if it is true.
Add netif_info("testing failed") into unhappy path of i40e_diag_test()

Fixes: e17bc411 ("i40e: Disable offline diagnostics if VFs are enabled")
Fixes: 510efb26 ("i40e: Fix ethtool offline diagnostic with netqueues")
Signed-off-by: Michal Jaron <michalx.jaron@intel.com>
Signed-off-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Tested-by: Gurucharan <gurucharanx.g@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>