io_uring: correct O_NONBLOCK check for splice punt

The splice file punt check uses file->f_mode to check for O_NONBLOCK,
but it should be checking file->f_flags. This leads to punting even
for files that have O_NONBLOCK set, which isn't necessary. This equates
to checking for FMODE_PATH, which will never be set on the fd in
question.

Fixes: 7d67af2c ("io_uring: add splice(2) support")
Signed-off-by: Jens Axboe <axboe@kernel.dk>