i40e: Fix call trace in setup_tx_descriptors

In the Linux kernel, the following vulnerability has been resolved:

i40e: Fix call trace in setup_tx_descriptors

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()