mm/swap_slots.c: remove always zero and unused return value of enable_swap_slots_cache()

enable_swap_slots_cache() always return zero and its return value is just
ignored by the caller.  So make enable_swap_slots_cache() void.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Link: https://lkml.kernel.org/r/20200924113554.50614-1-linmiaohe@huawei.com


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>