expose snapshot count via stat(2) of .zfs/snapshot (#13559)

Increase nlinks in stat results of ./zfs/snapshot based on snapshot
count. This provides quick and efficient method for administrators to
get snapshot counts without having to use libzfs or list the snapdir
contents.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Andrew Walker <awalker@ixsystems.com>
Closes #13559