Fix handling of explicit uuid

Fix a couple of bugs which show up if you try to explicitly set a
16-byte UUID when exporting a file system. First, exportfs cuts the
first two bytes off the UUID and writes something invalid to etab.
Second, mountd writes the _ascii_ form of the UUID to the kernel,
instead of converting it to hex.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: Steve Dickson <steved@redhat.com>