gfapi: indentify

Signed-off-by: Jens Axboe <axboe@fb.com>
Add ->invalidate() IO engine ops

Allow IO engines to plug in their own invalidate cache handler.
This fixes an issue on rbd, where we attempt to invalidate the
cache, but fail because it's not a valid file descriptor.

Reported-by: xan.peng@gmail.com
Signed-off-by: Jens Axboe <axboe@fb.com>