net: l3mdev: use obj-$(CONFIG_NET_L3_MASTER_DEV) form in net/Makefile

CONFIG_NET_L3_MASTER_DEV is a bool option. Change the ifeq conditional
to the standard obj-$(CONFIG_NET_L3_MASTER_DEV) form.

Use obj-y in net/l3mdev/Makefile because Kbuild visits this Makefile
only when CONFIG_NET_L3_MASTER_DEV=y.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20210125231659.106201-4-masahiroy@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>