LU-10391 lnet: allow ping packet to contain large nids

LU-10391 lnet: allow ping packet to contain large nids

The ping packet has an array of fixed-size status entries that only
have room for a 4-byte-address nid.

This patches adds a feature flag which activates a list of variable
sized entries after the initial array.

Each entry contains a 4-byte status and then a nid, rounded to a
multiple of 4 bytes.  The total number of bytes of the ping_info
(header, first array, subsequent list) is stored in the ns_unused
field of the first entry in the array.

The user-space interfaces only see the initial array.

Test-Parameters: trivial testlist=sanity-lnet
Test-Parameters: serverversion=2.12 serverdistro=el7.9 testlist=runtests
Test-Parameters: clientversion=2.12 testlist=runtests
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I774641d8cda24251337ce2d055caf05a14a9e088
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/44628
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Tested-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>