scoutfs: cow dirty radix blocks instead of moving

The radix allocator has to be careful to not get lost in recursion
trying to allocate metadata blocks for its dirty radix blocks while
allocating metadata blocks for others.

The first pass had used path data structures to record the references to
all the blocks we'd need to modify to reflect the frees and allocations
performed while dirtying radix blocks.  Once it had all the path blocks
it moved the old clean blocks into new dirty locations so that the
dirtying couldn't fail.

This had two very bad performance implications.  First, it meant that
trying to read clean versions of dirtied trees would always read the old
blocks again because their clean version had been moved to the dirty
version.  Typically this wouldn't happen but the server does exactly
this every time it tries to merge freed blocks back into its avail
allocator.  This created a significant IO load on the server.  Secondly,
that block cache move not being allowed to fail motivated us to move to
a locked rbtree for the block cache instead of the lockless rcu
radix_tree.

This changes the recursion avoidance to use per-block private metadata
to track every block that we allocate and cow rather than move.  Each
dirty block knows its parent ref and the blknos it would clear and set.
If dirtying fails we can walk back through all the blocks we dirty and
restore their original references before dropping all the dirty blocks
and returning an error.  This lets us get rid of the path structure
entirely and results in a much cleaner system.

This change meant tracking free blocks without clearing them as they're
used to satisfy dirty block allocations.  The change now has a cursor
that walks the avail metadata tree without modifying it.  While building
this it became clear that tracking the first set bits of refs doesn't
provide any value if we're always searching from a cursor.  The cursor
ends up providing the same value of avoiding constantly searching empty
initial bits and refs.  Maintaining the first metadata was just
overhead.

Signed-off-by: Zach Brown <zab@versity.com>

kmod/src/format.h

@@ -167,8 +167,6 @@ struct scoutfs_key {
 
 struct scoutfs_radix_block {
 	struct scoutfs_block_header hdr;
-	__le32 sm_first;
-	__le32 lg_first;
 	union {
 		struct scoutfs_radix_ref {
 			__le64 blkno;