bcachefs: btree node scan no longer uses btree cache

Previously, btree node scan used the btree node cache to check if btree
nodes were readable, but this is subject to interference from threads
scanning different devices trying to read the same node - and more
critically, nodes that we already attempted and failed to read before
kicking off scan.

Instead, we now allocate a 'struct btree' that does not live in the
btree node cache, and call bch2_btree_node_read_done() directly.

Cc: Nikita Ofitserov <himikof@gmail.com>
Reviewed-by: Nikita Ofitserov <himikof@gmail.com>
Reported-and-tested-by: Edoardo Codeglia <bcachefs@404.blue>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>