KTOR-7299 Fix close race condition causing descriptor to be closed while select is still busy

[Thomas-Vos]

KTOR-7299 IOException: Fail to select descriptor for ACCEPT

Unfortunately it does not resolve my specific issue, but I noticed this race condition when investigating the issue.
I cannot consistently reproduce this in a test so I did not add it, but I was able to reproduce it sometimes throwing kotlinx.io.IOException: Bad descriptor 5 for ACCEPT. This means the descriptor was closed multiple times, which should never happen. With the changes in this PR (and after #4637 is merged which I discovered due to this issue) it will throw kotlinx.io.IOException: Selectable closed instead. This is in line with JVM which throws ClosedChannelException (so IOException).

Explanation of race condition before this PR:

1. Closing selectormanager closes the closeQueue:

   ktor/ktor-network/nix/src/io/ktor/network/selector/SelectUtilsNix.kt

   Line 65 in e4fd251

   closeQueue.close()

2. any further calls to notifyClosed will call closeDescriptor because closeQueue is already closed:

   ktor/ktor-network/nix/src/io/ktor/network/selector/SelectUtilsNix.kt

   Lines 77 to 83 in e4fd251

   	actual fun notifyClosed(descriptor: Int) {
   	if (closeQueue.addLast(descriptor)) {
   	wakeupSignal.signal()
   	} else {
   	closeDescriptor(descriptor)
   	}
   	}

3. the above could happen while the selectionLoop is still active (because wakeupSignal.signal() could take some time). That means the descriptor is closed while select is still busy!

Fixed the above issue by moving closeQueue.close() to after the selection loop has finished so descriptors are not closed while select is still busy.

[e5l]

Hey @Thomas-Vos, thank you for the PR. LGTM