LU-13004 lnet: simplify ksock_tx.

LU-13004 lnet: simplify ksock_tx.

The tx_frags union in 'struct ksock_tx' is largely unnecessary.  The
payload is always lnet_kiov_t, the only kvec is a header.  So replace
the union with just those two fields.

Test-Parameters: trivial
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I2c4bd5397ddf412b11e9f741406de617c534c736
Reviewed-on: https://review.whamcloud.com/37850
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>