xfs: replace unnecessary seq_printf with seq_puts

Replace seq_printf with seq_puts when const string
in reference, which would avoid to deal with
unnecessary string format.

Signed-off-by: Zeng Heng <zengheng4@huawei.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Dave Chinner <david@fromorbit.com>