LU-14566 lnet: Skip discovery in LNetPrimaryNID if DD disabled

LU-14566 lnet: Skip discovery in LNetPrimaryNID if DD disabled

If discovery is disabled locally then the discovery thread will not
modify any peer objects as a result of the discovery process. Thus,
the primary NID of any peer we're asked to discover will not change
as a result of discovery. Therefore, we do not need to actually
perform discovery in LNetPrimaryNID() if discovery is disabled
locally. Since this routine can result in long client mount times
when a Lustre server is down we should avoid this unnecessary
discovery.

Lustre-change: https://review.whamcloud.com/43141
Lustre-commit: 16264da9e3c43a6368a25b6ded4113e8cfa57427

Test-Parameters: trivial
HPE-bug-id: LUS-9887
Signed-off-by: Chris Horn <chris.horn@hpe.com>
Change-Id: I6d188e16422ad47a146d52bb24cdd1b77a30aa71
Reviewed-on: https://review.whamcloud.com/43141
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/44224
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>