drbd: Fix drbd_request_prepare() discard handling

[ Upstream commit fad2d4ef636654e926d374ef038f4cd4286661f6 ]

Fix the test that verifies whether bio_op(bio) represents a discard
or write zeroes operation. Compile-tested only.

Cc: Philipp Reisner <philipp.reisner@linbit.com>
Cc: Lars Ellenberg <lars.ellenberg@linbit.com>
Fixes: 7435e901 ("drbd: zero-out partial unaligned discards on local backend")
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>