kunit: Fix 'hooks.o' build by recursing into kunit

KUnit's 'hooks.o' file need to be built-in whenever KUnit is enabled
(even if CONFIG_KUNIT=m).  We'd previously attemtped to do this by
adding 'kunit/hooks.o' to obj-y in lib/Makefile, but this caused hooks.c
to be rebuilt even when it was unchanged.

Instead, always recurse into lib/kunit using obj-y when KUnit is
enabled, and add the hooks there.

Fixes: 7170b7ed ("kunit: Add "hooks" to call into KUnit when it's built as a module").
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/linux-kselftest/CAHk-=wiEf7irTKwPJ0jTMOF3CS-13UXmF6Fns3wuWpOZ_wGyZQ@mail.gmail.com/Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/Makefile

@@ -127,14 +127,10 @@ CFLAGS_test_fpu.o += $(FPU_CFLAGS)
 
 obj-$(CONFIG_TEST_LIVEPATCH) += livepatch/
 
-obj-$(CONFIG_KUNIT) += kunit/
-# Include the KUnit hooks unconditionally. They'll compile to nothing if
-# CONFIG_KUNIT=n, otherwise will be a small table of static data (static key,
-# function pointers) which need to be built-in even when KUnit is a module.
-ifeq ($(CONFIG_KUNIT), m)
-obj-y += kunit/hooks.o
-else
-obj-$(CONFIG_KUNIT) += kunit/hooks.o
+# Some KUnit files (hooks.o) need to be built-in even when KUnit is a module,
+# so we can't just use obj-$(CONFIG_KUNIT).
+ifdef CONFIG_KUNIT
+obj-y += kunit/
 endif
 
 ifeq ($(CONFIG_DEBUG_KOBJECT),y)

lib/kunit/Makefile

@@ -13,7 +13,7 @@ kunit-objs +=				debugfs.o
 endif
 
 # KUnit 'hooks' are built-in even when KUnit is built as a module.
-lib-y +=				hooks.o
+obj-y +=				hooks.o
 
 obj-$(CONFIG_KUNIT_TEST) +=		kunit-test.o