filesetup: skip fallback invalidation with diskless ioengines

filesetup: skip fallback invalidation with diskless ioengines

Since commit 0bcf41cdc22dfee6b3f3b2ba9a533b4b103c70c2 ("io_u:
re-invalidate cache when looping around without file open/close") the
job:

./fio --ioengine=null --size=4k --bs=4k --rate=4k \
 --time_based --runtime=2s --name=badinvalidate

repeatedly produces the following warning while running:

fio: cache invalidation of badinvalidate.0.0 failed: Bad file descriptor

Fix this by skipping cache invalidation and logging a debug message for
diskless ioengines that don't define an explicit invalidation op.

Signed-off-by: Sitsofe Wheeler <sitsofe@yahoo.com>