fs: don't pass NULL dev_desc to most filesystems

FAT and ext4 expect that the passed in block device descriptor not be
NULL. This causes problems on sandbox, where get_device_and_partition()
succeeds for the "host" device, yet passes back a NULL device descriptor.
Add special handling for this situation, so that the generic filesystem
commands operate as expected on sandbox.

Signed-off-by: Stephen Warren <swarren@nvidia.com>