io_uring: don't duplicate ->file check in sfr

IORING_OP_SYNC_FILE_RANGE is marked as .needs_file, so the common path
will take care of assigning and validating req->file, no need to
duplicate it in io_sfr_prep().

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>