Recompute the owner of /proc/PID/{fd,fdinfo} and fd/N on every access.

[copybara-service]

Recompute the owner of /proc/PID/{fd,fdinfo} and fd/N on every access.

The fd and fdinfo directories and the fd/N symlinks took their owner from the
task's credentials when the inode was created. /proc/PID entries are cached,
so a task that was looked up while root and then switched to another uid kept
a root-owned, mode 0500 /proc/PID/fd, and processes running as the new uid got
EACCES listing its fds. setpriv does exactly this (libcap-ng reads
/proc/self/status before setresuid), which breaks tools that map sockets to
processes through socket:[N] links (ss -p, lsof, fuser) when run as the
service user.

Linux recomputes the owner of /proc/PID entries on every revalidation
(fs/proc/base.c:pid_revalidate -> pid_update_inode -> task_dump_owner), and of
fd/N in fs/proc/fd.c:tid_fd_revalidate. The fd/N symlinks and the fdinfo
directory are now taskOwnedInodes like the rest of /proc/PID. The fd directory
keeps its same-thread-group permission exception and computes its owner the
same way.

Wrapping these inodes made them implement kernel.TaskOwnedInode, so
pidfd_send_signal and setns on them took the /proc/PID pidfd path and failed
with ESRCH. Files under /proc/PID are now rejected as on Linux:
pidfd_send_signal returns EBADF, and setns falls back to the nsfs check and
returns EINVAL. This also fixes the errno for files such as /proc/PID/status.

Tests: proc_test ProcPid.FdOwnerFollowsSetuid fails on runsc before this
change and passes on Linux. pidfd_test ProcPidFdinfoIsNotAPidfd,
ProcPidFdLinkIsNotAPidfd and ProcPidStatusIsNotAPidfd check the errnos.