LU-13004 socklnd: discard tx_iov.

LU-13004 socklnd: discard tx_iov.

tx_iov always points to tx_hdr, so we can discard tx_iov, and just use
&tx_hdr.

Test-Parameters: trivial
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I9738b6f953184fdea859da7a9c4187227fa61143
Reviewed-on: https://review.whamcloud.com/37851
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>