netfs: fix sense of DIO test on short read

netfs: fix sense of DIO test on short read

The sense of this test is reversed. There's nothing that prevents
userland from requesting a DIO read that is longer than the available
data. Conversely, we don't expect a buffered read to be short unless it
hits the EOF.

Suggested-by: David Howells <dhowells@redhat.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>