bcachefs: Fix validate_bset() repair path

When we truncate a bset (due to it extending past the end of the btree
node), we can't skip the rest of the validation for e.g. the packed
format (if it's the first bset in the node).

Reported-by:  <syzbot+4d722d3c539d77c7bc82@syzkaller.appspotmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>