net: liquidio: fix mixed module-builtin object

With CONFIG_LIQUIDIO=m and CONFIG_LIQUIDIO_VF=y (or vice versa),
$(common-objs) are linked to a module and also to vmlinux even though
the expected CFLAGS are different between builtins and modules.

This is the same situation as fixed by commit 637a642f ("zstd:
Fixing mixed module-builtin objects").

Introduce the new module, liquidio-core, to provide the common functions
to liquidio and liquidio-vf.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>