btrfs: move fs root init stuff into btrfs_init_fs_root

We have a helper for reading fs roots that just reads the fs root off
the disk and then sets REF_COWS and init's the inheritable flags.  Move
this into btrfs_init_fs_root so we can later get rid of this helper and
consolidate all of the fs root reading into one helper.

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