mm/memtest: add underflow detection for size calculation

The computation:

	end = start + (size - (start_phys_aligned - start_phys)) / incr

could theoretically underflow if size < offset, leading to a massive
iteration.

Add VM_WARN_ON_ONCE() to detect cases where the region size is smaller
than the alignment offset. While this should never happen in practice
due to memblock guarantees, the warning helps catch potential bugs in
early memory initialization code.

Suggested-by: Mike Rapoport <rppt@kernel.org>
Signed-off-by: Kevin Lourenco <klourencodev@gmail.com>
Link: https://patch.msgid.link/20251229161321.9079-1-klourencodev@gmail.com
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

Author: kevinLourencoDev

mm/memtest.c

@@ -50,6 +50,8 @@ static void __init memtest(u64 pattern, phys_addr_t start_phys, phys_addr_t size
 	start_bad = 0;
 	last_bad = 0;
 
+	VM_WARN_ON_ONCE(size < start_phys_aligned - start_phys);
+
 	for (p = start; p < end; p++)
 		WRITE_ONCE(*p, pattern);