ila: show usage even if the module is not available

ila: show usage even if the module is not available

Currently, the `ip ila` command tries to initialize a genl context
even when we just want to see the help for the command, which doesn't
require to talk to the kernel at all.

Delay genl initialization, which can fail if the module isn't loaded,
until the point where we will actually need it.

Fixes: ec71cae0bb7b ("ila: Support for configuring ila to use netfilter hook")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>