bcachefs: Check for journal entries overruning end of sb clean section

In the Linux kernel, the following vulnerability has been resolved: bcachefs: Check for journal entries overruning end of sb clean section Fix a missing bounds check in superblock validation. Note that we don't yet have repair code for this case - repair code for individual items is generally low priority, since the whole superblock is checksummed, validated prior to write, and we have backups.