lock vm around rb_free_generic_ivar

Currently, this can be reproduced by:

r = Ractor.new do
    a = [1, 2, 3]
    a.object_id
    a.dup # this frees the generic ivar for `object_id` on the copied object
    :done
end
r.take

In debug builds, this hits an assertion failure without this fix.

Author: luke-gruber

bootstraptest/test_ractor.rb

@@ -2319,6 +2319,16 @@ def initialize(a)
   'ok'
 }
 
+# take vm lock when deleting generic ivars from the global table
+assert_equal 'ok', %q{
+  Ractor.new do
+    a = [1, 2, 3]
+    a.object_id
+    a.dup # this deletes generic ivar on dupped object
+    'ok'
+  end.take
+}
+
 # There are some bugs in Windows with multiple threads in same ractor calling ractor actions
 # Ex: https://github.com/ruby/ruby/actions/runs/14998660285/job/42139383905
 unless /mswin/ =~ RUBY_PLATFORM

variable.c

@@ -1273,15 +1273,19 @@ rb_free_generic_ivar(VALUE obj)
 
     bool too_complex = rb_shape_obj_too_complex_p(obj);
 
-    if (st_delete(generic_fields_tbl_no_ractor_check(obj), &key, &value)) {
-        struct gen_fields_tbl *fields_tbl = (struct gen_fields_tbl *)value;
+    RB_VM_LOCK_ENTER();
+    {
+        if (st_delete(generic_fields_tbl_no_ractor_check(obj), &key, &value)) {
+            struct gen_fields_tbl *fields_tbl = (struct gen_fields_tbl *)value;
 
-        if (UNLIKELY(too_complex)) {
-            st_free_table(fields_tbl->as.complex.table);
-        }
+            if (UNLIKELY(too_complex)) {
+                st_free_table(fields_tbl->as.complex.table);
+            }
 
-        xfree(fields_tbl);
+            xfree(fields_tbl);
+        }
     }
+    RB_VM_LOCK_LEAVE();
 }
 
 size_t