LU-10391 socklnd: move lnet_hdr unpack into ->pro_unpack

LU-10391 socklnd: move lnet_hdr unpack into ->pro_unpack

Converting the lnet_hdr from network-format to host-format
is currently done in ksocknal_process_recv().
Move it to ->pro_unpack() so that a different protocol
can send it in a different format.

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: Icc22f4b52c1391d382c28bad157795f5477f4d7c
Reviewed-on: https://review.whamcloud.com/43609
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>