[illumos-Developer] webrev: 1282 nsmb_open has a race

[Gordon Ross]

Please review:  Relatively simple bug found by inspection.
  http://www.yalms.org/cr/illumos-1282/

Also cleaned up nsmb_attach, which had inherited some code
from nfs to make minor unit zero busy.  Completely unnecessary.
and a minor nit about getinfo...

While I haven't seen this race cause damage, I'm a little worried
that my prior changes in 1121 could make this more exposed.

Thanks,
Gordon