Reintroduce IO accounting on zvols on Linux 3.19+

zfsonlinux/zfs@e20cd6f7a8922709b1aa2ecefd783390102d79e0 caused us to
lose IO accounting on zvols. When I originally wrote that last year, the
symbols we needed to maintain IO accounting were GPL exported, but
torvalds/linux@394ffa503bc40e32d7f54a9b817264e81ce131b4 provided
suitable symbols for restoring this functionality 4 months later. We
can call them to restore the IO accounting on Linux 3.19 and later as
well as any older kernels where that patch is backported.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3741