LU-13004 lnet: remove lnet_extract_iov()

LU-13004 lnet: remove lnet_extract_iov()

The only place this is called, the src kvec is
NULL with length 0, so it returns 0.
So remove it.

Test-Parameters: trivial
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I72fd39d0b4330fd4f811f2ee1476a653407855b4
Reviewed-on: https://review.whamcloud.com/37848
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>