kho: warn and exit when unpreserved page wasn't preserved

Calling __kho_unpreserve() on a pair of (pfn, end_pfn) that wasn't
preserved is a bug.  Currently, if that is done, the physxa or bits can be
NULL.  This results in a soft lockup since a NULL physxa or bits results
in redoing the loop without ever making any progress.

Return when physxa or bits are not found, but WARN first to loudly
indicate invalid behaviour.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: fc33e4b ("kexec: enable KHO support for memory preservation")
Signed-off-by: Pratyush Yadav <[email protected]>
Reviewed-by: Mike Rapoport (Microsoft) <[email protected]>
Cc: Alexander Graf <[email protected]>
Cc: Baoquan He <[email protected]>
Cc: Pasha Tatashin <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: prati0100

kernel/kexec_handover.c

@@ -171,12 +171,12 @@ static void __kho_unpreserve(struct kho_mem_track *track, unsigned long pfn,
 		const unsigned long pfn_high = pfn >> order;
 
 		physxa = xa_load(&track->orders, order);
-		if (!physxa)
-			continue;
+		if (WARN_ON_ONCE(!physxa))
+			return;
 
 		bits = xa_load(&physxa->phys_bits, pfn_high / PRESERVE_BITS);
-		if (!bits)
-			continue;
+		if (WARN_ON_ONCE(!bits))
+			return;
 
 		clear_bit(pfn_high % PRESERVE_BITS, bits->preserve);