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

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.

Change-Id: I5d9de7edf05dec142138a1d49b9025cb017340d0
Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>