From: "B. D. Elliott" <bde@nwlink.com>

There's a bug:

- Someone reads a (say) root-owned process's /proc/pid/fd directory

  The inodes are instantiated owned by root.

- That process does a setuid

- The /proc/pid/* files still have the old ownerships.

This happened because we are now caching the proc entries.

The patch rewrites the ownership of the inodes under /proc/pid in the
d_revalidate() handler.