Backout priterator() for walking allprocess list.

Backout priterator() for walking allprocess list.

This approach does not work as LIST_NEXT() of a removed element
does not return NULL.  I causes a crash in syzcaller and triggers
kernel diagnostic assertion "vp->v_uvcount == 0" in sys/kern/kern_unveil.c
line 845 during reboot.  Unfortunately the backout brings back the
race in fill_file() and fstat(1) may crash the kernel.

Reported-by: syzbot+54fba1c004d7383d5e85@syzkaller.appspotmail.com