Fix string/index variables being unflexed by default

Fix string/index variables being unflexed by default

I got the status backward (B_FALSE for fixed, rather than B_TRUE for
flex); before:

  $ zfs get mountpoint tarta-zoot -r
  NAME                                 PROPERTY    VALUE       SOURCE
  tarta-zoot                           mountpoint  /           local
  tarta-zoot/PAGEFILE.SYS              mountpoint  -           -
  tarta-zoot/etc                       mountpoint  /etc        inherited from tarta-zoot
  tarta-zoot/home                      mountpoint  /home       inherited from tarta-zoot
  tarta-zoot/home/xspon                mountpoint  /home/xspon  inherited from tarta-zoot
  tarta-zoot/home/nabijaczleweli       mountpoint  /home/nabijaczleweli  inherited from tarta-zoot
  tarta-zoot/home/nabijaczleweli/tftp  mountpoint  /home/nabijaczleweli/tftp  inherited from tarta-zoot
  tarta-zoot/home/root                 mountpoint  /root       local

after:

  $ zfs get mountpoint tarta-zoot -r
  NAME                                 PROPERTY    VALUE                      SOURCE
  tarta-zoot                           mountpoint  /                          local
  tarta-zoot/PAGEFILE.SYS              mountpoint  -                          -
  tarta-zoot/etc                       mountpoint  /etc                       inherited from tarta-zoot
  tarta-zoot/home                      mountpoint  /home                      inherited from tarta-zoot
  tarta-zoot/home/xspon                mountpoint  /home/xspon                inherited from tarta-zoot
  tarta-zoot/home/nabijaczleweli       mountpoint  /home/nabijaczleweli       inherited from tarta-zoot
  tarta-zoot/home/nabijaczleweli/tftp  mountpoint  /home/nabijaczleweli/tftp  inherited from tarta-zoot
  tarta-zoot/home/root                 mountpoint  /root                      local

Fixes: be8e1d81bf2b85f6f76851b7013ac54e9bf488f0 ("Flex
 non-pretty-printed properties and raw-/pretty-print remaining ones")
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13286