bcachefs: bch2_btree_node_scrub()

bcachefs: bch2_btree_node_scrub()

Add a function for scrubbing btree nodes - reading them in, and kicking
off a rewrite if there's an error.

The btree_node_read_done() checks have to be duplicated because we're
not using a pointer to a struct btree - the btree node might already be
in cache, and we need to check a specific replica, which might not be
the one we previously read from.

This will be used in the next patch implementing high-level scrub.

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