psi: add struct psi_group_ext

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I8BCV4



-------------------------------

Change struct psi_group directly will causes the kabi broken.
Therefore, add a new struct psi_group_ext for new variables, which
will be added in the next patch of pressure.stat.

Signed-off-by: Lu Jialin <lujialin4@huawei.com>