mm/swap: remove unneeded return value of free_swap_slot

mm/swap: remove unneeded return value of free_swap_slot

The return value of free_swap_slot is always 0 and also ignored now.
Remove it to clean up the code.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Miaohe Lin <[email protected]>
Reviewed-by: David Hildenbrand <[email protected]>
Reviewed-by: Oscar Salvador <[email protected]>
Cc: Alistair Popple <[email protected]>
Cc: David Howells <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: NeilBrown <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>