bcachefs: Add missing validation for superblock section clean

We were forgetting to check for jset entries that overrun the end of the
section - both in validate and to_text(); to_text() needs to be safe for
types that fail to validate.

Reported-by:  <syzbot+c48865e11e7e893ec4ab@syzkaller.appspotmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>