LU-17379 lnet: add LNetPeerDiscovered to LNet API

LU-17379 lnet: add LNetPeerDiscovered to LNet API

LNetPeerDiscovered is added to allow lustre check
whether the peer has been successfully discovered by LNet
before attempting to open a connection to it.
For example, given a mount command with a list of NIDs,
Lustre can use LNetAddPeer API to initiate discovery on
every candidate first, and later use LNetPeerDiscovered
to select a reachable peer to connect to.

Test-Parameters: trivial testlist=sanity-lnet
Signed-off-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Change-Id: I7c9964148a5a2a24d7889b8b4c2e488a433ca258
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/53926
Reviewed-by: Frank Sehr <fsehr@whamcloud.com>
Reviewed-by: Mikhail Pershin <mpershin@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>