async_xor: check src_offs is not NULL before updating it

When PAGE_SIZE is greater than 4kB, multiple stripes may share the same
page. Thus, src_offs is added to async_xor_offs() with array of offsets.
However, async_xor() passes NULL src_offs to async_xor_offs(). In such
case, src_offs should not be updated. Add a check before the update.

Fixes: ceaf296(async_xor: increase src_offs when dropping destination page)
Cc: [email protected] # v5.10+
Reported-by: Oleksandr Shchirskyi <[email protected]>
Tested-by: Oleksandr Shchirskyi <[email protected]>
Signed-off-by: Xiao Ni <[email protected]>
Signed-off-by: Song Liu <[email protected]>

Author: XiaoNi87

crypto/async_tx/async_xor.c

@@ -233,7 +233,8 @@ async_xor_offs(struct page *dest, unsigned int offset,
 		if (submit->flags & ASYNC_TX_XOR_DROP_DST) {
 			src_cnt--;
 			src_list++;
-			src_offs++;
+			if (src_offs)
+				src_offs++;
 		}
 
 		/* wait for any prerequisite operations */