Specifically, the following was broken:

$ mount -t procfs procfs /proc
$ ls -l /proc

rS351741 reworked readdir slightly to avoid pfs_node/pidhash LOR, but inadvertently regressed pid == NO_PID; new pfs_lookup_proc() fails for the obvious reasons, and later pfs_visible_proc doesn't capture the pid == NO_PID -> return 1 aspect of pfs_visible. We can infact skip this whole block if we're operating on a NO_PID directory, as it's always visible.