bcachefs: Repair pass for scanning for btree nodes

If a btree root or interior btree node goes bad, we're going to lose a
lot of data, unless we can recover the nodes that it pointed to by
scanning.

Fortunately btree node headers are fully self describing, and
additionally the magic number is xored with the filesytem UUID, so we
can do so safely.

This implements the scanning - next patch will rework topology repair to
make use of the found nodes.

Signed-off-by: Kent Overstreet <[email protected]>

fs/bcachefs/Makefile

@@ -17,6 +17,7 @@ bcachefs-y		:=	\
 	btree_journal_iter.o	\
 	btree_key_cache.o	\
 	btree_locking.o		\
+	btree_node_scan.o	\
 	btree_trans_commit.o	\
 	btree_update.o		\
 	btree_update_interior.o	\

fs/bcachefs/bcachefs.h

@@ -456,6 +456,7 @@ enum bch_time_stats {
 
 #include "alloc_types.h"
 #include "btree_types.h"
+#include "btree_node_scan_types.h"
 #include "btree_write_buffer_types.h"
 #include "buckets_types.h"
 #include "buckets_waiting_for_journal_types.h"
@@ -1103,6 +1104,8 @@ struct bch_fs {
 	struct journal_keys	journal_keys;
 	struct list_head	journal_iters;
 
+	struct find_btree_nodes	found_btree_nodes;
+
 	u64			last_bucket_seq_cleanup;
 
 	u64			counters_on_mount[BCH_COUNTER_NR];