Revert "bcachefs: Delete some obsolete journal_seq_blacklist code"

This reverts commit f95b61228efd04c9c158123da5827c96e9773b29.

It turns out, we're seeing filesystems in the wild end up with
blacklisted btree node bsets - this should not be happening, and until
we understand why and fix it we need to keep this code around.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

bcachefs: Delete some obsolete journal_seq_blacklist code

Since metadata version bcachefs_metadata_version_btree_ptr_sectors_written,
we haven't needed the journal seq blacklist mechanism for ignoring
blacklisted btree node writes - we now only need it for ignoring journal
entries that were written after the newest flush journal entry, and then
we only need to keep those blacklist entries around until journal replay
is finished.

That means we can delete the code for scanning btree nodes to GC
journal_seq_blacklist entries.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

bcachefs: Fix a bug with the journal_seq_blacklist mechanism

Previously, we would start doing btree updates before writing the first
journal entry; if this was after an unclean shutdown, this could cause
those btree updates to not be blacklisted.

Also, move some code to headers for userspace debug tools.

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

bcachefs: Rewrite journal_seq_blacklist machinery

Now, we store blacklisted journal sequence numbers in the superblock,
not the journal: this helps to greatly simplify the code, and more
importantly it's now implemented in a way that doesn't require all btree
nodes to be visited before starting the journal - instead, we
unconditionally blacklist the next 4 journal sequence numbers after an
unclean shutdown.

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

bcachefs: Initial commit

Initially forked from drivers/md/bcache, bcachefs is a new copy-on-write
filesystem with every feature you could possibly want.

Website: https://bcachefs.org

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