btrfs_trans_handle->root is documented as for use for confirming
that the root passed in to start the transaction is the same as the
one ending it.  It's used in several places when an fs_info pointer
is needed, so let's just add an fs_info pointer directly.  Eventually,
the root pointer can be removed.
Signed-off-by: NJeff Mahoney <jeffm@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>