ata: scsi: rename flag ATA_QCFLAG_FAILED to ATA_QCFLAG_EH

The name ATA_QCFLAG_FAILED is misleading since it does not mean that a
QC completed in error, or that it didn't complete at all. It means that
libata decided to schedule EH for the QC, so the QC is now owned by the
libata error handler (EH).

The normal execution path is responsible for not accessing a QC owned
by EH. libata core enforces the rule by returning NULL from
ata_qc_from_tag() for QCs owned by EH.

It is quite easy to mistake that a QC marked with ATA_QCFLAG_FAILED was
an error. However, a QC that was actually an error is instead indicated
by having qc->err_mask set. E.g. when we have a NCQ error, we abort all
QCs, which currently will mark all QCs as ATA_QCFLAG_FAILED. However, it
will only be a single QC that is an error (i.e. has qc->err_mask set).

Rename ATA_QCFLAG_FAILED to ATA_QCFLAG_EH to more clearly highlight that
this flag simply means that a QC is now owned by EH. This new name will
not mislead to think that the QC was an error (which is instead
indicated by having qc->err_mask set).

This also makes it more obvious that the EH code skips all QCs that do
not have ATA_QCFLAG_EH set (rather than ATA_QCFLAG_FAILED), since the EH
code should simply only care about QCs that are owned by EH itself.

Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
Reviewed-by: John Garry <john.g.garry@oracle.com>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>