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

BugLink: http://bugs.launchpad.net/bugs/1650607
commit a0ac402cfcdc904f9772e1762b3fda112dcc56a0 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>