[GC] Fix freelist crash - don't modify during check-only operation (#…

…104992)

Backport of #104876 to release/8.0-staging

/cc @markples

## Customer Impact

- [x] Customer reported
- [ ] Found internally

GC can crash in region code if GC.RegisterForFullGCNotification is used.

## Regression

- [x] Yes
- [ ] No

Regression is part of general move to regions, but specific code was introduced in #48691

## Testing

The behavior is very difficult to repro and likely hadn't occurred before because it relies on a race condition, a region allocation failing, and GC.RegisterForFullGCNotification be used.  I built a custom GC that always triggers the region allocation failure path and was able to repro and verify the fix in GCPerfSim.

## Risk

Low: only impacts GC.RegisterForFullGCNotification path; only impacts notifications; may cause a notification for a full GC to be skipped, but only in a case where no other reasons for full gc occur -and- the code would be subject to the race condition and crash

src/coreclr/gc/gc.cpp

@@ -21595,10 +21595,13 @@ int gc_heap::generation_to_condemn (int n_initial,
     }
 
 #ifdef USE_REGIONS
-    if (!try_get_new_free_region())
+    if (!check_only_p)
     {
-        dprintf (GTC_LOG, ("can't get an empty region -> full compacting"));
-        last_gc_before_oom = TRUE;
+        if (!try_get_new_free_region())
+        {
+            dprintf (GTC_LOG, ("can't get an empty region -> full compacting"));
+            last_gc_before_oom = TRUE;
+        }
     }
 #endif //USE_REGIONS