mm: swap: change swap_alloc_slow() to void

mm: swap: change swap_alloc_slow() to void

swap_alloc_slow() does not need to return a bool, as all callers handle
allocation results via the entry parameter.  Update the function signature
and remove return statements accordingly.

Link: https://lkml.kernel.org/r/20251031065011.40863-5-youngjun.park@lge.com
Signed-off-by: Youngjun Park <youngjun.park@lge.com>
Reviewed-by: Kairui Song <kasong@tencent.com>
Reviewed-by: Baoquan He <bhe@redhat.com>
Acked-by: Chris Li <chrisl@kernel.org>
Cc: Barry Song <baohua@kernel.org>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>