LU-4863 lloop: avoid panic during blockdev_info

LU-4863 lloop: avoid panic during blockdev_info

Change the LL_IOC_LLOOP_INFO ioctl in the lustre lloop
device driver to return an error instead of causing
panics with LASSERT().

Signed-off-by: Bob Glossman <bob.glossman@intel.com>
Change-Id: Ib50afa94960dee6fb580038d6e96ea648252d9ea
Reviewed-on: http://review.whamcloud.com/9888
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>