Default NO_INSTALLEXTRAKERNELS to yes, not no

The old (^/stable/9) default was yes, not no ("no" was the new default
introduced recently that broke POLA). Restore it to keep POLA like
glebius intended in r299077

MFC after: 3 days
X-MFC with: r299086
Pointyhat to: ngie (research before assuming and committing next time)
Sponsored by: EMC / Isilon Storage Division