[PATCH -V6 15/21] swap: Support to copy PMD swap mapping when fork()

During fork, the page table need to be copied from parent to child. A PMD swap mapping need to be copied too and the swap reference count need to be increased. When the huge swap cluster has been split already, we need to split the PMD swap mapping and fallback to PTE copying. When swap count

[PATCH -V6 15/21] swap: Support to copy PMD swap mapping when fork()

During fork, the page table need to be copied from parent to child. A PMD swap mapping need to be copied too and the swap reference count need to be increased. When the huge swap cluster has been split already, we need to split the PMD swap mapping and fallback to PTE copying. When swap count