bcachefs: Simplify, fix bch2_backpointer_get_key()

 - backpointer_not_found() checks backpointers_no_use_write_buffer, no
   need to do it inbackpointer_get_key().

 - always use backpointer_get_node() for pointers to nodes:
   backpointer_get_key() was sometimes returning the key from the root
   node unlocked.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>