bcachefs: Fix b->written overflow

When bset past end of btree node, we should not add sectors to
b->written, which will overflow b->written.

Reported-by:  <syzbot+3cb3d9e8c3f197754825@syzkaller.appspotmail.com>
Tested-by:  <syzbot+3cb3d9e8c3f197754825@syzkaller.appspotmail.com>
Signed-off-by: Alan Huang <mmpgouride@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>