When we switch to multiple global trees we'll need to access the
appropriate extent root depending on the block group or possibly root.
To handle this, use a helper in most places and then the actual root in
places where it is required. We will whittle down the direct accessors
with future patches, but this does the bulk of the preparatory work.
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>