bcachefs: Fix null ptr deref in btree_path_lock_root()

Historically, we required that all btree node roots point to a valid
(possibly fake) node, but we're improving our ability to continue in the
presence of errors.

Reported-by:  <syzbot+e22007d6acb9c87c2362@syzkaller.appspotmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>