epoll: Store epoll_event udata member in ext member of kevent.

[wulf]

Summary

Current epoll implementation stores udata fields of epoll_event
structure in special dynamically-sized table rather than in udata field
of backing kevent structure because of 2 reasons:

1. Kevent's udata size is smaller than epoll's on 32-bit archs.
2. Kevent's udata can be clobbered on execution EPOLL_CTL_ADD as kqueue modifies existing event while epoll returns error in this case.

After r320043 has introduced 4 new 64bit user data members (ext[]),
we can store epoll udata in one of them and drop aforementioned table.
According to kqueue_register() source code ext members are not updated
when existing kevent is modified that fixes p.2.

As a side effect the patch fixes PR/252582.