scsi: target: prefer dbroot of /etc/target over /var/target

The target database root directory, dbroot, has defaulted to /var/target
for a while, but its main client, targetcli-fb, has been moving it to
/etc/target for quite some time. With the plethora of target drivers now
appearing, it has become more difficult to initialize this attribute
before use by any child drivers.

If the directory /etc/target exists, use that as the DB root. Otherwise,
fall back to using /var/target.

The ability to override this dbroot attribute still exists via sysfs.

Signed-off-by: Lee Duncan <lduncan@suse.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>