ceph: cleanup hardcoded constants of file handle size

The ceph/export.c contains very confusing logic of file handle size
calculation based on hardcoded values. This patch makes the cleanup of
this logic by means of introduction the named constants.

Signed-off-by: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>
Reviewed-by: Alex Markuze <amarkuze@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>