LU-10391 lnet: discard lnet_sock_accept()

LU-10391 lnet: discard lnet_sock_accept()

There is no longer any important difference between
lnet_sock_accept(), and kernel_accept(..., O_NONBLOCK).
So remove lnet_sock_accept().

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: Iad7c91abe2359758982e3740a21c91232c919aa0
Reviewed-on: https://review.whamcloud.com/37303
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alexey Lyashkov <alexey.lyashkov@hpe.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>