sys/net: only panic on unset tx_csum_flags if cap is disabled

[corvink]

Summary

isc_capabilites contains all capabilities of the device. isc_capenable
contains all capabilities which should be enabled. If the TXCSUM should
be disabled, it's valid to set tx_csum_flags to zero.

[kbowling]

What is triggering this for you?

The invariant is designed to catch a driver bug where the driver expresses TXCSUM capability but does not define what isc_tx_csum_flags it supports (see i.e. recent e1000 lem fix). It shouldn't occur that you have the capability but no isc_tx_csum_flags.

[erj]

I'm going to agree with @kbowling here -- it looks like it's a driver bug / reasonable to fail if the driver sets IFCAP_TXCSUM without setting the CSUM_* bits that it supports for transmit checksumming.

[gallatin]

I think I agree with Eric and Kevin after looking at this further, and withdraw my approval

[corvink]

Got confused by the meaning of isc_capabilites and isc_capenable. Sry for the noise.