LU-15944 lnet: remove crash with UDSP

LU-15944 lnet: remove crash with UDSP

The following sequence of commands caused a crash:
  # lnetctl udsp add --dst tcp --prio 1
  # lnetctl discover 192.168.122.60@tcp
Pointer to lnet_peer_net in udsp_info is checked before used.

Comments about syntax of "lnetctl udsp" command were updated.

Test-Parameters: trivial testlist=lnet-selftest,sanity-lnet
Signed-off-by: Cyril Bordage <cbordage@whamcloud.com>
Change-Id: Ie3ae40b184e22627655e7f3813c5d16d38a6cfb8
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/48801
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Chris Horn <chris.horn@hpe.com>
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Reviewed-by: Frank Sehr <fsehr@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>