epoll: Do not search again in EPOLLONESHOT case

The f_lock is not dropped. No need to searth the key in the map again.

Signed-off-by: Asias He <asias@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
epoll: Update comments in epoll::wait

To reflect the current locking scheme.

Signed-off-by: Asias He <asias@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>