Btrfs: fix metadata dirty throttling limits

Once a metadata block has been written, it must be recowed, so the
btrfs dirty balancing call has a check to make sure a fair amount of metadata
was actually dirty before it started writing it back to disk.

A previous commit had changed the dirty tracking for metadata without
updating the btrfs dirty balancing checks.  This commit switches it
to use the correct counter.

Signed-off-by: Chris Mason <chris.mason@oracle.com>