Share dynamic char dev major for /proc/gvisor/{checkpoint,fscheckpoint}

Procfs mounts used a per-mount major number for /proc/gvisor/checkpoint.
Since Linux reserves a limited number of dynamic major numbers, running
many isolated container actions concurrently would exhaust the pool and
crash the sentry with EBUSY.

This adds VFS.GetSharedDynamicCharDevMajor(key) to allocate a major on
first use and share it across all subsequent mounts for that key.
The sentry now consumes exactly two majors for these devices regardless
of how many procfs instances are mounted.

PiperOrigin-RevId: 967985107