nfs: fix a minor do_div portability issue

This change modifies filelayout_get_dense_offset() to use the functions
in math64.h and thus avoid a 32-bit platform compile error trying to
use do_div() on an s64 type.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>
Reviewed-by: Boaz Harrosh <bharrosh@panasas.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>