LU-10391 lnet: change various function to return pointer

LU-10391 lnet: change various function to return pointer

Rather than returning an error code, and using a by-ref
arg to return the socket pointer, the functions
 lnet_connect()
 lnet_sock_listen()
 lnet_sock_connect()
now return the socket pointer or an ERR_PTR() with the
error encoded.
This is more in-keeping with kernel style.

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: Ic09d3bf38a89920a1298848d5c8a9017428cb752
Reviewed-on: https://review.whamcloud.com/37701
Reviewed-by: Aurelien Degremont <degremoa@amazon.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>