btrfs: move root tree prototypes to their own header

Move all the root-tree.c prototypes to root-tree.h, and then update all
the necessary files to include the new header.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>