[PATCH] libata-eh-fw: use special reserved tag and qc for internal commands

New EH may issue internal commands to recover from error while failed
qc's are still hanging around.  To allow such usage, reserve tag
ATA_MAX_QUEUE-1 for internal command.  This also makes it easy to tell
whether a qc is for internal command or not.  ata_tag_internal() test
implements this test.

To avoid breaking existing drivers, ata_exec_internal() uses
ATA_TAG_INTERNAL only for drivers which implement ->error_handler.
For drivers using old EH, tag 0 is used.  Note that this makes
ata_tag_internal() test valid only when ->error_handler is
implemented.  This is okay as drivers on old EH should not and does
not have any reason to use ata_tag_internal().

Signed-off-by: Tejun Heo <htejun@gmail.com>