orangefs: silence harmless integer overflow warning

The issue here is that in orangefs_bufmap_alloc() we do:

	bufmap->buffer_index_array 		kzalloc(DIV_ROUND_UP(bufmap->desc_count, BITS_PER_LONG), GFP_KERNEL);

If we choose a bufmap->desc_count like -31 then it means the
DIV_ROUND_UP ends up having an integer overflow. The result is that
kzalloc() returns the ZERO_SIZE_PTR and there is a static checker warning.

But this bug is harmless because on the next lines we use ->desc_count to do a kcalloc(). That has integer overflow checking built in so the kcalloc() fails and we return an error code.

Anyway, it doesn't make sense to talk about negative sizes and blocking them silences the static checker warning.

index 75375e9..6333cbb 100644
@@ -344,6 +344,11 @@ int orangefs_bufmap_initialize(struct ORANGEFS_dev_map_desc *user_desc)
 		     user_desc->size,
 		     user_desc->count);
 
+	if (user_desc->total_size < 0 ||
+	    user_desc->size < 0 ||
+	    user_desc->count < 0)
+		goto out;
+
 	/*
 	 * sanity check alignment and size of buffer that caller wants to
 	 * work with