Make SCSI SG v4 driver enabled by default and remove EXPERIMENTAL dependency, since udev depends on BSG

Make Block Layer SG support v4 the default, since recent udev versions
depend on this to access serial numbers and other low level info properly.

This should be backported to older kernels as well, since most distros have
enabled this for a long time.
Signed-off-by: John Stoffel <john@stoffel.org>
Cc: stable@kernel.org
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>