ashmem: account for older kernel which don't have vma_set_anonymous

Was first introduced in 4.18 and needs to be called from ashmem when
running with a kernel >= 4.18.

ashmem/ashmem.c

@@ -419,9 +419,13 @@ static int ashmem_mmap(struct file *file, struct vm_area_struct *vma)
 			fput(asma->file);
 			goto out;
 		}
+#if LINUX_VERSION_CODE >= KERNEL_VERSION(4, 18, 0)
 	} else {
 		vma_set_anonymous(vma);
 	}
+#else
+	}
+#endif
 
 	if (vma->vm_file)
 		fput(vma->vm_file);