udf: Avoid using corrupted block bitmap buffer

In the Linux kernel, the following vulnerability has been resolved: udf: Avoid using corrupted block bitmap buffer When the filesystem block bitmap is corrupted, we detect the corruption while loading the bitmap and fail the allocation with error. However the next allocation from the same bitmap will notice the bitmap buffer is already loaded and tries to allocate from the bitmap with mixed results (depending on the exact nature of the bitmap corruption). Fix the problem by using BH_verified bit to indicate whether the bitmap is valid or not.