Several factors may cause the active watchpoint count to be lesser
than the poll table size as a result of collecting events:

- a stale file descriptor is encountered (-EBADF)
- bad user memory is written to while copying back evl_poll_event (-EFAULT)
- the user event set has fewer entries than the ready set (> 0)

In all of these cases, we may end up watching fewer files than the
total amount of items in the poll table, in which case poll_context.nr
is larger than the actual number of active watchpoints.

For this reason, clear_wait() cannot iterate over poll_context.nr
items, but should rather consider the active watchpoints only. To this
end, introduce poll_context.active which is set by collect_events()
appropriately.

Signed-off-by: Philippe Gerum <rpm@xenomai.org>