LU-12410 lnet: Define enum for lnetctl commands

LU-12410 lnet: Define enum for lnetctl commands

The enum values can be used to faciliate code sharing amongst the
lnetctl routines.

Test-Parameters: trivial
Signed-off-by: Chris Horn <hornc@cray.com>
Change-Id: I1085a70a17aefa300f3bf949cf867b2712131a0f
Reviewed-on: https://review.whamcloud.com/35504
Reviewed-by: Shaun Tancheff <stancheff@cray.com>
Reviewed-by: Petros Koutoupis <pkoutoupis@cray.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>