LoongArch: Define a default value for VM_DATA_DEFAULT_FLAGS

This is a trivial cleanup, commit c62da0c3 ("mm/vma: define a
default value for VM_DATA_DEFAULT_FLAGS") has unified default values of
VM_DATA_DEFAULT_FLAGS across different platforms.

Apply the same consistency to LoongArch.

Suggested-by: Wentao Guan <guanwentao@uniontech.com>
Signed-off-by: Yuli Wang <wangyuli@uniontech.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>