vfs, swap: compile out IS_SWAPFILE() on swapless configs

No swap support -- no swapfiles possible.

Signed-off-by: Alexey Dobriyan (Yandex) <adobriyan@gmail.com>
Link: https://lore.kernel.org/r/2391c7f5-0f83-4188-ae56-4ec7ccbf2576@p183


Signed-off-by: Christian Brauner <brauner@kernel.org>