splice: simplify a conditional in copy_splice_read

Check for -EFAULT instead of wrapping the check in an ret < 0 block.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Reviewed-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/20230614140341.521331-3-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>