In an upcoming patch bch2_backpointer_get_key() will be repairing when
it finds a dangling backpointer; it will need to flush the btree write
buffer before it can definitively say there's an error.

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

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

bch_backpointer no longer contains the bucket_offset field, it's just a
direct LBA mapping (with low bits to account for compressed extent
splitting), so we don't need to refer to the device to construct it
anymore.

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

New on disk format version: backpointers new include the generation
number of the bucket they refer to, and the obsolete bucket_offset field
(no longer needed because we no longer store backpointers in alloc keys)
is gone.

This is an expensive forced upgrade - hopefully the last; we have to run
the extents_to_backpointers recovery pass to regenerate backpointers.

It's a forced incompatible upgrade because the alternative would've been
permamently making backpointers bigger, and as one of the biggest btrees
(along with the extents btree) that's not an ideal option.

It's worth it though, because this allows us to make the
check_extents_to_backpointers pass drastically cheaper: an upcoming
patch changes it to sum up backpointers in a bucket and check the sum
against the sector counts for that bucket, only looking for missing
backpointers if they don't match (and then only for specific buckets).

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

Add a new parameter to bkey validate functions, and use it to improve
invalid bkey error messages: we can now print the btree and depth it
came from, or if it came from the journal, or is a btree root.

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

bch_backpointer.bucket_offset is going away - it's no longer needed
since we no longer store backpointers in alloc keys, the same
information is in the key position itself.

And we'll be reclaiming the space in bch_backpointer for the bucket
generation number.

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

Since we no longer store backpointers in alloc keys, there's no reason
not to pass around bkey_i_backpointers; this means we don't have to pass
the bucket pos separately.

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

Better helpers for iterating over backpointers within a specific bucket

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

_noerror means don't produce inconsistent errors, so it should be using
bch2_dev_rcu_noerror().

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

Since for quite some time backpointers have only been stored in the
backpointers btree, not alloc keys (an aborted experiment, support for
which has been removed) - we can replace get_next_backpointer() with
simple btree iteration.

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