bcachefs: Fix sb_field_downgrade validation

In the Linux kernel, the following vulnerability has been resolved: bcachefs: Fix sb_field_downgrade validation - bch2_sb_downgrade_validate() wasn't checking for a downgrade entry extending past the end of the superblock section - for_each_downgrade_entry() is used in to_text() and needs to work on malformed input; it also was missing a check for a field extending past the end of the section