sg_write()/bsg_write() is not fit to be called under KERNEL_DS

sg_write()/bsg_write() is not fit to be called under KERNEL_DS

Orabug: 25340071
CVE: CVE-2016-10088

Both damn things interpret userland pointers embedded into the payload;
worse, they are actually traversing those.  Leaving aside the bad
API design, this is very much _not_ safe to call with KERNEL_DS.
Bail out early if that happens.

Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
(cherry picked from commit 128394eff343fc6d2f32172f03e24829539c5835)
Signed-off-by: Brian Maly <brian.maly@oracle.com>