btrfs: Don't BUG_ON errors from btrfs_create_subvol_root()

This is called from only one place - create_subvol() which passes errors
safely back out to it's caller, btrfs_mksubvol where they are handled.

Additionally, btrfs_create_subvol_root() itself bug's needlessly from error
return of btrfs_update_inode(). Since create_subvol() was fixed to catch
errors we can bubble this one up too.

Signed-off-by: Mark Fasheh <mfasheh@suse.com>