bcachefs: Fix missing validation for bch_backpointer.level

This fixes an assertion pop where we try to navigate to the target of
the backpointer, and the path level isn't what we expect.

Reported-by:  <syzbot+b17df21b4d370f2dc330@syzkaller.appspotmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>