dm raid1: always return error if all legs fail

If all mirror legs fail, always return an error instead of holding the
bio, even if the handle_errors option was set.  At present it is the
responsibility of the driver underneath us to deal with retries,
multipath etc.

The patch adds the bio to the failures list instead of holding it
directly.  do_failures tests first if all legs failed and, if so,
returns the bio with -EIO.  If any leg is still alive and handle_errors
is set, do_failures calls hold_bio.

Reviewed-by: Takahiro Yasui <tyasui@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>