[SCSI] osduld: Ref-counting bug fix

If scsi has released the device (logout), and exofs has last
reference on the osduld_device it will be freed by
osd_uld_release() within the call to fput(). But this will
oops in cdev_release() which is called after the fops->release.
(cdev is embedded within osduld_device). __uld_get/put pair
makes sure we have a cdev for the duration of fput()

Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>