libnvdimm: use generic iostat interfaces

nd_iostat_start() and nd_iostat_end() implement the same functionality
that generic_start_io_acct() and generic_end_io_acct() already provide.

Change nd_iostat_start() and nd_iostat_end() to call the generic iostat
interfaces.  There is no change in the nd interfaces.

Signed-off-by: Toshi Kani <toshi.kani@hpe.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>