selftests: Fix memfd to support relocatable build (O=objdir)

Fix memfd to support relocatable build (O=objdir). This calls out
source files necessary to build tests and simplfies the dependency
enforcement.

Tested the following:

Note that cross-build for fuse_mnt has dependency on -lfuse.

make all
make clean
make kselftest-install O=/arm64_build/ ARCH=arm64 HOSTCC=gcc \
CROSS_COMPILE=aarch64-linux-gnu- TARGETS=memfd
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/memfd/Makefile

@@ -4,9 +4,8 @@ CFLAGS += -I../../../../include/uapi/
 CFLAGS += -I../../../../include/
 CFLAGS += -I../../../../usr/include/
 
-TEST_GEN_PROGS := memfd_test
+TEST_GEN_PROGS := memfd_test fuse_test fuse_mnt
 TEST_PROGS := run_fuse_test.sh run_hugetlbfs_test.sh
-TEST_GEN_FILES := fuse_mnt fuse_test
 
 fuse_mnt.o: CFLAGS += $(shell pkg-config fuse --cflags)
 
@@ -14,7 +13,7 @@ include ../lib.mk
 
 $(OUTPUT)/fuse_mnt: LDLIBS += $(shell pkg-config fuse --libs)
 
-$(OUTPUT)/memfd_test: memfd_test.c common.o
-$(OUTPUT)/fuse_test: fuse_test.c common.o
+$(OUTPUT)/memfd_test: memfd_test.c common.c
+$(OUTPUT)/fuse_test: fuse_test.c common.c
 
-EXTRA_CLEAN = common.o
+EXTRA_CLEAN = $(OUTPUT)/common.o