LU-13004 lnet: remove msg_iov from lnet_msg.

LU-13004 lnet: remove msg_iov from lnet_msg.

This field is never set, so remove it and remove all uses of it.

Conversely, msg_kiov is always non-NULL, so remove code that depends
on it being NULL.

Test-Parameters: trivial
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I4c5e98bd54a463ed62ea8d06b9781aa8b4e96f50
Reviewed-on: https://review.whamcloud.com/37844
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>