Remove GENHD_FL_DRIVERFS

This flag is not used, so best discarded.

Signed-off-by: NeilBrown <neilb@suse.de>
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Hi Jens,
 I came across this recently - these are the only two occurances
 of "GENHD_FL_DRIVERFS" in the kernel, so it cannot be needed.
NeilBrown
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>