ksmbd: replace SUBDIRS with M in package recipe

The SUBDIRS variable has been removed in kernel 5.4, and was deprecated
since the beginnig of kernel git history in favour of M or KBUILD_EXTMOD.

Signed-off-by: David Bauer <mail@david-bauer.net>

kernel/ksmbd/Makefile

@@ -60,7 +60,7 @@ EXTRA_CFLAGS += -DCONFIG_SMB_INSECURE_SERVER=1
 endif
 
 define Build/Compile
-	$(KERNEL_MAKE) SUBDIRS="$(PKG_BUILD_DIR)" \
+	$(KERNEL_MAKE) M="$(PKG_BUILD_DIR)" \
 	EXTRA_CFLAGS="$(EXTRA_CFLAGS)" \
 	$(PKG_EXTRA_KCONFIG) \
 	CONFIG_SMB_SERVER=m \