LU-11911 lov: fix lov_iocontrol for inactive OST case

For inactive OSTs lov->lov_tgts[index]->ltd_exp is
NULL. lov_iocontrol() is to check that before dereferencing to
lov->lov_tgts[index]->ltd_exp->exp_obd.

Signed-off-by: Vladimir Saveliev <c17830@cray.com>
Cray-bug-id: LUS-6937
Test-Parameters: trivial
Change-Id: I4bb332ee2c50b07a1471035556f4d77a3559847f
Reviewed-on: https://review.whamcloud.com/34148
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alexandr Boyko <c17825@cray.com>