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/[email protected]
Signed-off-by: Cyrill Gorcunov <[email protected]>
Cc: Andrey Vagin <[email protected]>
Cc: Pavel Emelyanov <[email protected]>
Cc: Michael Kerrisk <[email protected]>
Cc: Shuah Khan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>