Read vfs2 epoll events atomically.

Discovered by ayushranjan@:

VFS2 was employing the following algorithm for fetching ready events from an
epoll instance:
- Create a statically sized EpollEvent slice on the stack of size 16.
- Pass that to EpollInstance.ReadEvents() to populate.
   - EpollInstance.ReadEvents() requeues level-triggered events that it returns
     back into the ready queue.
- Write the results to usermem.
- If the number of results were = 16 then recall EpollInstance.ReadEvents() in
  the hopes of getting more. But this will cause duplication of the "requeued"
  ready level-triggered events.

So if the ready queue has >= 16 ready events, the EpollWait for loop will spin
until it fills the usermem with `maxEvents` events.

Fixes #3521

PiperOrigin-RevId: 331840527

0 files changed, 0 insertions, 0 deletions