LU-12595 lnet: Return EHOSTUNREACH for unreachable gateway

LU-12595 lnet: Return EHOSTUNREACH for unreachable gateway

Commit 43b35351e9ca258773e89c2d68047e939fb822fb contains a flaw in
that it shouldn't be a fatal error to encounter an unreachable
gateway when parsing routes. Parsing should continue in case there
are any valid, reachable routes that are being added. Returning EINAL
here will cause a failure to load the LNet module. lnet_parse_route()
explicitly allows for lnet_add_route() to return EHOSTUNREACH for
just this purpose.

Lustre-change: https://review.whamcloud.com/35630
Lustre-commit: 7c12c24c8a10be2d0ad005d6d99d97cee6bcde18

Test-parameters: trivial
Fixes: 43b35351e9 ("LU-12411 lnet: Do not allow gateways on remote nets")
Signed-off-by: Chris Horn <hornc@cray.com>
Change-Id: Ia0f28779a3505eff02dafdc23a6e01c1d0cbc84b
Reviewed-by: Alexey Lyashkov <c17817@cray.com>
Reviewed-by: Amir Shehata <ashehata@whamcloud.com>
Signed-off-by: Minh Diep <mdiep@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/36871
Tested-by: Maloo <maloo@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>