mm/kfence: remove unnecessary CONFIG_KFENCE option

In mm/Makefile has:

  obj-$(CONFIG_KFENCE) += kfence/

So that we don't need 'obj-$(CONFIG_KFENCE) :=' in mm/kfence/Makefile,
delete it from mm/kfence/Makefile.

Link: https://lkml.kernel.org/r/20220221065525.21344-1-tangmeng@uniontech.comSigned-off-by: tangmeng <tangmeng@uniontech.com>
Reviewed-by: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Dmitriy Vyukov <dvyukov@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/kfence/Makefile

 # SPDX-License-Identifier: GPL-2.0
 
-obj-$(CONFIG_KFENCE) := core.o report.o
+obj-y := core.o report.o
 
 CFLAGS_kfence_test.o := -g -fno-omit-frame-pointer -fno-optimize-sibling-calls
 obj-$(CONFIG_KFENCE_KUNIT_TEST) += kfence_test.o