LU-11542 import: fix race between imp_state & imp_invalid

LU-11542 import: fix race between imp_state & imp_invalid

We set import to LUSTRE_IMP_DISCON and then deactive when
it is unreplayable. Someone may set this import up between
those two operations. So we will get a invalid import with
FULL state.

Signed-off-by: Yang Sheng <ys@whamcloud.com>
Change-Id: Ib4cec0bcaf6f4b221ba260edb94749a4e523f5e6
Reviewed-on: https://review.whamcloud.com/33395
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Wang Shilong <wshilong@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>