PIIX4_SMBHSTSTAT_ERR can be set for several reasons that, unfortunately,
cannot be distinguished, but the most typical case is a missing or hung
slave (SMB_ENOACK).
PIIX4_SMBHSTSTAT_FAIL means failed or killed / aborted transaction, so
it's previous mapping to SMB_ENOACK was not ideal.
After this change an smb(4) access to a missing slave results in ENXIO
rather than EIO. To me, that seems to be more appropriate.
MFC after: 3 weeks
4c2fb03954