bpfilter: Allow to build bpfilter_umh as a module without static library

Originally, bpfilter_umh was linked with -static only when
CONFIG_BPFILTER_UMH=y.

Commit 8a2cc050 ("bpfilter: use 'userprogs' syntax to build
bpfilter_umh") silently, accidentally dropped the CONFIG_BPFILTER_UMH=y
test in the Makefile. Revive it in order to link it dynamically when
CONFIG_BPFILTER_UMH=m.

Since commit b1183b6d ("bpfilter: check if $(CC) can link static
libc in Kconfig"), the compiler must be capable of static linking to
enable CONFIG_BPFILTER_UMH, but it requires more than needed.

To loosen the compiler requirement, I changed the dependency as follows:

    depends on CC_CAN_LINK
    depends on m || CC_CAN_LINK_STATIC

If CONFIG_CC_CAN_LINK_STATIC in unset, CONFIG_BPFILTER_UMH is restricted
to 'm' or 'n'.

In theory, CONFIG_CC_CAN_LINK is not required for CONFIG_BPFILTER_UMH=y,
but I did not come up with a good way to describe it.

Fixes: 8a2cc050 ("bpfilter: use 'userprogs' syntax to build bpfilter_umh")
Reported-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Tested-by: Michal Kubecek <mkubecek@suse.cz>
Link: https://lore.kernel.org/bpf/20200701092644.762234-1-masahiroy@kernel.org

net/bpfilter/Kconfig

@@ -9,12 +9,14 @@ menuconfig BPFILTER
 if BPFILTER
 config BPFILTER_UMH
 	tristate "bpfilter kernel module with user mode helper"
-	depends on CC_CAN_LINK_STATIC
+	depends on CC_CAN_LINK
+	depends on m || CC_CAN_LINK_STATIC
 	default m
 	help
 	  This builds bpfilter kernel module with embedded user mode helper
 
-	  Note: your toolchain must support building static binaries, since
-	  rootfs isn't mounted at the time when __init functions are called
-	  and do_execv won't be able to find the elf interpreter.
+	  Note: To compile this as built-in, your toolchain must support
+	  building static binaries, since rootfs isn't mounted at the time
+	  when __init functions are called and do_execv won't be able to find
+	  the elf interpreter.
 endif

net/bpfilter/Makefile

@@ -7,10 +7,12 @@ userprogs := bpfilter_umh
 bpfilter_umh-objs := main.o
 userccflags += -I $(srctree)/tools/include/ -I $(srctree)/tools/include/uapi
 
+ifeq ($(CONFIG_BPFILTER_UMH), y)
 # builtin bpfilter_umh should be linked with -static
 # since rootfs isn't mounted at the time of __init
 # function is called and do_execv won't find elf interpreter
 userldflags += -static
+endif
 
 $(obj)/bpfilter_umh_blob.o: $(obj)/bpfilter_umh