LU-12678 socklnd: don't fall-back to tcp_sendpage.

LU-12678 socklnd: don't fall-back to tcp_sendpage.

sk_prot->sendpage is never NULL, so there is no
need for a fallback to tcp_sendpage.

Test-Parameters: trivial testlist=sanity-lnet
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: Iaabf47790f2809fe98a0f09da31aa441021b26ab
Reviewed-on: https://review.whamcloud.com/39134
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>