classifier: Rename struct cls_table as cls_subtable.

The naming of the classifier table has been a source of confusion,
since each OpenFlow table is implemented as a classifier, which
consists of multiple (sub)tables.  This name change hopefully makes
classifier related discussion a bit less confusing.

For consistency, relevant field names as well as the function and
variable names have been renamed in similar fashion.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>