xfs: sort variable alphabetically to avoid repeated declaration

Variable 'xfs_agf_buf_ops', 'xfs_agi_buf_ops', 'xfs_dquot_buf_ops' and
'xfs_symlink_buf_ops' are declared twice, so sort these variables
alphabetically and remove the repeated declaration.

Cc: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Shaokun Zhang <zhangshaokun@hisilicon.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>