genetlink: inline genl_get_cmd()

All callers go via genl_get_cmd_split() now, so rename it
to genl_get_cmd() remove the original.

Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>