[PATCH] libata: fix oops caused rescanning NULL sdev

Depending on timing, ata_scsi_dev_rescan() might encounter a device
which is enabled but not yet attached to sdev.  On such cases, the
original code caused oops.  This patch makes ata_scsi_dev_rescan()
rescan only device which are attached to sdevs.

While at it, properly indent leading comment and add description about
how it's synchronized with sdev attach/detach.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>