Replace swap_dirty_idx function with built-in one,
because swap_dirty_idx does only a simple byte to byte swap.

Since Spectre mitigations have made indirect function calls more
expensive, and the default simple byte copies swap is implemented
without them, an "optimized" custom swap function is now
a waste of time as well as code.
Signed-off-by: Andrey Abramov <st5pub@yandex.ru>
Reviewed by: George Spelvin <lkml@sdf.org>
Signed-off-by: Richard Weinberger <richard@nod.at>