ceph: use 64-bit math in ceph_calc_file_object_mapping()

ceph: use 64-bit math in ceph_calc_file_object_mapping()

Convert ceph_calc_file_object_mapping() so it uses 64-bit variables
and divide operators.  This is in anticipation of the next patch,
to keep the the changes it incurs more focused.

I made most of the variable names more verbose, and because it now
uses do_div() some of the calculations get done in a different
order.  Net result is that the code does what it did before, but it
looks quite a bit different...

Signed-off-by: Alex Elder <elder@dreamhost.com>