[PATCH -V6 02/21] swap: Add __swap_duplicate_locked()

The part of __swap_duplicate() with lock held is separated into a new function __swap_duplicate_locked(). Because we will add more logic about the PMD swap mapping into __swap_duplicate() and keep the most PTE swap mapping related logic in __swap_duplicate_locked(). Just mechanical code

[PATCH -V6 02/21] swap: Add __swap_duplicate_locked()

The part of __swap_duplicate() with lock held is separated into a new function __swap_duplicate_locked(). Because we will add more logic about the PMD swap mapping into __swap_duplicate() and keep the most PTE swap mapping related logic in __swap_duplicate_locked(). Just mechanical code