Use the swap() macro to simplify the code and improve its readability.

Fixes the following Coccinelle/coccicheck warning reported by
swap.cocci:

  WARNING opportunity for swap()

Compile-tested only.
Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>