LU-10391 socklnd: Change ksock_hello_msg to struct lnet_nid

LU-10391 socklnd: Change ksock_hello_msg to struct lnet_nid

'struct ksock_hello_msg' now stores 'struct lnet_nid', but it is
converted to 'struct ksock_hello_msg_nid4' - the old format - for
transmit, which is converted back on receive.

This opens the way for a new version of the hello protocol
which will use 16byte addresses.

Test-Parameters: trivial
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: I22e86f9088f6001f203f24f93ef292fcf2a8e69f