Don't feed anything but regular iovec's to blk_rq_map_user_iov

Don't feed anything but regular iovec's to blk_rq_map_user_iov

In theory we could map other things, but there's a reason that function
is called "user_iov".  Using anything else (like splice can do) just
confuses it.

Reported-and-tested-by: Johannes Thumshirn <jthumshirn@suse.de>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit a0ac402cfcdc904f9772e1762b3fda112dcc56a0)

Orabug: 25230657
CVE: CVE-2016-9576
Signed-off-by: Dhaval Giani <dhaval.giani@oracle.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Conflicts:
	block/blk-map.c