tools/testing/selftests/kcmp/kcmp_test.c: add KCMP_EPOLL_TFD testing

tools/testing/selftests/kcmp/kcmp_test.c: add KCMP_EPOLL_TFD testing

KCMP's KCMP_EPOLL_TFD mode merged in commit 0791e3644e5ef2 ("kcmp: add
KCMP_EPOLL_TFD mode to compare epoll target files") we've had no selftest
for it yet (except in criu development list).  Thus add it.

Link: http://lkml.kernel.org/r/20170901151620.GK1898@uranus.lan
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Andrey Vagin <avagin@openvz.org>
Cc: Pavel Emelyanov <xemul@virtuozzo.com>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>