bcachefs: Fix btree for nonexistent tree depth

The fix for when we should increase tree depth in journal replay was
entirely bogus.

We should only increase the tree depth in journal replay when recovery
from btree node scan, and then only for keys found by btree node scan.

This needs additional work - we should be shooting down existing
interior node pointers when recovery from scan, they shouldn't be
showing up here.

Fixes: b47a82ff ("bcachefs: Only run 'increase_depth' for keys from btree node csan")
Cc: Alan Huang <mmpgouride@gmail.com>
Reported-by:  <syzbot+8deb6ff4415db67a9f18@syzkaller.appspotmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>