Allow longer SPA names in stats

The pool name can be 256 chars long. Today, in /proc/spl/kstat/zfs/
the name is limited to < 32 characters. This change is to allows
bigger pool names.

Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: gaurkuma <gauravk.18@gmail.com>
Closes #6481