nfs-mount-monitor: strengthen fix_mount recovery sequence

Add lazy umount fallback, D-state process killer, stunnel restart, and
60-second hard deadline to prevent fix_mount from looping forever when
processes are stuck in D state on a stale NFSv4-over-stunnel mount.

Recovery sequence is now:
1. mount -o remount -f  (cheap, no disruption)
2. kill_pinning_processes (SIGKILL D-state procs with nfs_ wchan)
3. umount -f            (force unmount)
4. umount -l            (lazy detach VFS node if -f failed)
5. systemctl restart stunnel + 2s sleep (refresh TLS transport)
6. mount                (fresh mount)

The 60s deadline uses bash $SECONDS so fix_mount can never outlast its
own 10-second timer interval by an unbounded amount. Deployed to all
three r-nodes (r0/r1/r2) via rex nfs_mount_monitor.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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