Revert "kern: RACCT: Keep process credentials alive via references"
The change causes a panic on boot with INVARIANTS kernels. Revert for
now.
This reverts commit a5d1a0c9bfcca38528b861c5afb51ea9b1696b65.
Reported by: syzbot+74624c6fcbb384ea0113@syzkaller.appspotmail.com