xen/blkback: Check for proper operation.

The patch titled: "xen/blkback: Fix the inhibition to map pages
when discarding sector ranges." had the right idea except that
it used the wrong comparison operator. It had == instead of !=.

This fixes the bug where all (except discard) operations would
have been ignored.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>