ubifs: Fix cut and paste error on sb type comparisons

The check for the bad node type of sb->type is checking sa->type
and not sb-type. This looks like a cut and paste error. Fix this.

Detected by PVS-Studio, warning: V581

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Richard Weinberger <richard@nod.at>