mm/swap: don't BUG_ON() due to uninitialized swap slot cache

This BUG_ON() triggered for me once at shutdown, and I don't see a
reason for the check.  The code correctly checks whether the swap slot
cache is usable or not, so an uninitialized swap slot cache is not
actually problematic afaik.

I've temporarily just switched the BUG_ON() to a WARN_ON_ONCE(), since
I'm not sure why that seemingly pointless check was there.  I suspect
the real fix is to just remove it entirely, but for now we'll warn about
it but not bring the machine down.

Cc: "Huang, Ying" <[email protected]>
Cc: Tim Chen <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/swap_slots.c

@@ -267,7 +267,7 @@ int free_swap_slot(swp_entry_t entry)
 {
 	struct swap_slots_cache *cache;
 
-	BUG_ON(!swap_slot_cache_initialized);
+	WARN_ON_ONCE(!swap_slot_cache_initialized);
 
 	cache = &get_cpu_var(swp_slots);
 	if (use_swap_slot_cache && cache->slots_ret) {