lpm: merge tbl24 and tbl8 structures

lpm: merge tbl24 and tbl8 structures

The tbl8 and tbl24 structures were essentially identical except for
slightly different names for one or two fields. Merge these two
structures into a single structure definition.

Two fields have been renamed as part of this change: the
"ext_entry" field in the tbl24 has been renamed to "valid_group" to match
the tbl8 value to make the merge easier, and the "tbl8_gindex" field has
been renamed to "group_idx". The "valid_group" field now serves two
purposes: in a tbl8 it indicates if the group, i.e. the tbl8, is valid,
and in a tbl24, it indicates if the "group_idx" is valid, i.e. whether
the value is a next_hop or a tbl8 index. [The name "group_idx" was used
to make this latter link between the fields clearer]

Suggested-by: Vladimir Medvedkin <medvedkinv@gmail.com>
Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>