LU-10391 lnet: use data_ready callback to trigger accept()

LU-10391 lnet: use data_ready callback to trigger accept()

Rather than blocking in lnet_sock_accept(), set up a data_ready
callback, and use that to find out when to call lnet_sock_accept()
again.

This simplifies lnet_sock_accept() (which will be removed in
next patch), and means that we could listen on multiple
sockets, which will be useful for IPv6 support.

The code design in based on that in net/sunrpc/svcsock.c.

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I3015f2f6b6d420af5c8454b6c1a99611b48e7702
Reviewed-on: https://review.whamcloud.com/37302
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Alexey Lyashkov <alexey.lyashkov@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>