db/view/view_update_from_staging_generator: Break semaphore on stop()
db/view/view_update_from_staging_generator: Break semaphore on stop()

This avoid having fibers waiting _registration_sem without ever being
notified.

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

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