proc_fuse: add psi(pressure stall information) procfs

[loyou]

Kernel support psi(pressure stall information) since 4.20
with procfs /proc/pressure/{io,cpu,memory} and
cgroupv2 {io.pressure, cpu.pressure, memory.pressure}.

This patch add read-only psi procfs,
and people can get pressure information now.
Full functional feature for monitoring are still under investigation.

[mihalicyn]

Hey @loyou,

thanks for working on this.

Please, notice that you have a failing test https://github.com/lxc/lxcfs/actions/runs/11809711932/job/32900481717?pr=665

[loyou]

I'm a little confused with the test result,

==> Testing /sys/devices/system/cpu/online
../src/lxcfs.c: 910: lxcfs_read: unknown file type: path=/sys/devices/system/cpu/online, type=-1, fi->fh=105965433156896
cat: /tmp/tmp.tDXXvo8T0e/sys/devices/system/cpu/online: Invalid argument
../src/lxcfs.c: 988: lxcfs_release: unknown file type: path=/sys/devices/system/cpu/online, type=-1, fi->fh=105965433156896

the failed line number does not match the patched file lines,
and cat /sys/devices/system/cpu/online in lxc container with patched lxcfs, it works fine.
I will resolve the live upgrade issue first and then check it again.