Increment generation which LFC is disabled by assigning 0 to neon.file_cache_size_limit (#6692)

## Problem

test_lfc_resize sometimes filed with assertion failure when require lock
in write operation:

```
	if (lfc_ctl->generation == generation)
	{
		Assert(LFC_ENABLED());
```

## Summary of changes

Increment generation when 0 is assigned to neon.file_cache_size_limit

## Checklist before requesting a review

- [ ] I have performed a self-review of my code.
- [ ] If it is a core feature, I have added thorough tests.
- [ ] Do we need to implement analytics? if so did you add the relevant
metrics to the dashboard?
- [ ] If this PR requires public announcement, mark it with
/release-notes label and add several sentences in this section.

## Checklist before merging

- [ ] Do not forget to reformat commit message to not include the above
checklist

Co-authored-by: Konstantin Knizhnik <knizhnik@neon.tech>

pgxn/neon/file_cache.c

@@ -314,6 +314,9 @@ lfc_change_limit_hook(int newval, void *extra)
 		lfc_ctl->used -= 1;
 	}
 	lfc_ctl->limit = new_size;
+	if (new_size == 0) {
+		lfc_ctl->generation += 1;
+	}
 	neon_log(DEBUG1, "set local file cache limit to %d", new_size);
 
 	LWLockRelease(lfc_lock);