db/view/build_progress_virtual_reader: Fix full ck detection
db/view/build_progress_virtual_reader: Fix full ck detection

As an optimization, the virtual reader doesn't change the underlying
key if it is not full, and hence doesn't include the extra clustering
key. However, this detection is broken because it checked for 3
clustering columns, instead of 2.

This patch fixes that by obtaining the clustering key size from the
underlying schema instead of hardcoding the size.

Signed-off-by: Duarte Nunes <a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>

avikivity/scylla,scylladb/scylla,duarten/scylla,duarten/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla