net: mellanox: add DEVLINK dependencies

The new NET_DEVLINK infrastructure can be a loadable module, but the drivers
using it might be built-in, which causes link errors like:

drivers/net/built-in.o: In function `mlx4_load_one':
:(.text+0x2fbfda): undefined reference to `devlink_port_register'
:(.text+0x2fc084): undefined reference to `devlink_port_unregister'
drivers/net/built-in.o: In function `mlxsw_sx_port_remove':
:(.text+0x33a03a): undefined reference to `devlink_port_type_clear'
:(.text+0x33a04e): undefined reference to `devlink_port_unregister'

There are multiple ways to avoid this:

a) add 'depends on NET_DEVLINK || !NET_DEVLINK' dependencies
   for each user
b) use 'select NET_DEVLINK' from each driver that uses it
   and hide the symbol in Kconfig.
c) make NET_DEVLINK a 'bool' option so we don't have to
   list it as a dependency, and rely on the APIs to be
   stubbed out when it is disabled
d) use IS_REACHABLE() rather than IS_ENABLED() to check for
   NET_DEVLINK in include/net/devlink.h

This implements a variation of approach a) by adding an
intermediate symbol that drivers can depend on, and changes
the three drivers using it.
Signed-off-by: NArnd Bergmann <arnd@arndb.de>
Fixes: 09d4d087 ("mlx4: Implement devlink interface")
Fixes: c4745500 ("mlxsw: Implement devlink interface")
Acked-by: NJiri Pirko <jiri@mellanox.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/infiniband/hw/mlx4/Kconfig

 config MLX4_INFINIBAND
 	tristate "Mellanox ConnectX HCA support"
 	depends on NETDEVICES && ETHERNET && PCI && INET
+	depends on MAY_USE_DEVLINK
 	select NET_VENDOR_MELLANOX
 	select MLX4_CORE
 	---help---

drivers/net/ethernet/mellanox/mlx4/Kconfig

@@ -4,6 +4,7 @@
 
 config MLX4_EN
 	tristate "Mellanox Technologies 1/10/40Gbit Ethernet support"
+	depends on MAY_USE_DEVLINK
 	depends on PCI
 	select MLX4_CORE
 	select PTP_1588_CLOCK

drivers/net/ethernet/mellanox/mlxsw/Kconfig

@@ -4,6 +4,7 @@
 
 config MLXSW_CORE
 	tristate "Mellanox Technologies Switch ASICs support"
+	depends on MAY_USE_DEVLINK
 	---help---
 	  This driver supports Mellanox Technologies Switch ASICs family.
 

net/Kconfig

@@ -403,6 +403,15 @@ config NET_DEVLINK
 	  infrastructure to support access to physical chip-wide config and
 	  monitoring.
 
+config MAY_USE_DEVLINK
+	tristate
+	default m if NET_DEVLINK=m
+	default y if NET_DEVLINK=y || NET_DEVLINK=n
+	help
+	  Drivers using the devlink infrastructure should have a dependency
+	  on MAY_USE_DEVLINK to ensure they do not cause link errors when
+	  devlink is a loadable module and the driver using it is built-in.
+
 endif   # if NET
 
 # Used by archs to tell that they support BPF_JIT