LU-14661 lnet: Provide kernel API for adding peers

Implement LNetAddPeer() API to allow other kernel modules to add
peers to LNet.

Peers created via this API are not marked as having been configured
by DLC. As such, they can be overwritten by discovery.

Test-Parameters: trivial
HPE-bug-id: LUS-9293
Signed-off-by: Chris Horn <chris.horn@hpe.com>
Change-Id: Ibb057f702ea29d60233fbd1680d8caec98064d5d
Reviewed-on: https://review.whamcloud.com/43509
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Reviewed-by: Alexander Boyko <alexander.boyko@hpe.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>