bcachefs: Tweak btree cache helpers for use by btree node scan

btree node scan needs to not use the btree node cache: that causes
interference from prior failed reads and parallel workers.

Instead we need to allocate btree nodes that don't live in the btree
cache, so that we can call bch2_btree_node_read_done() directly.

This patch tweaks the low level helpers so they don't touch the btree
cache lists.

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>