eventpoll: ensure we pass back -EBADF for a bad file descriptor

eventpoll: ensure we pass back -EBADF for a bad file descriptor

A previous commit moved file checking earlier in epoll_ctl(), and as a
result, we now have error == -EINVAL going into the test case for
whether or not the passed in target file is valid or not. This should be
-EBADF as per earlier, assign it correctly, otherwise epoll_ctl02 from
LTP correctly identifies this bad return value:

epoll_ctl02.c:87: TFAIL: epoll_clt(...) if fd is an invalid fd expected EBADF: EINVAL (22)

Fixes: 3fe64a3a96d4 ("eventpoll: move file checking earlier for epoll_ctl()")
Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
Tested-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>