LU-15136 socklnd: default conns_per_peer to 0

Setting conns_per_peer to 0 triggers socklnd to choose the
(heuristically) optimal setting for the interface given its speed.
Make 0 the default for socklnd conns_per_peer.

Test-parameters: trivial testlist=sanity-lnet

Signed-off-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Fixes: c44afcfb72 ("LU-12815 socklnd: set conns_per_peer based on link speed")
Change-Id: Ie6e76eaee8693472384cce362b394b216142884e
Reviewed-on: https://review.whamcloud.com/45319
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Chris Horn <chris.horn@hpe.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>