ceph: fix up the types of the file layout helpers

ceph: fix up the types of the file layout helpers

The helper macros for accessing file layout fields of an on-disk
ceph file layout structure cast their results to type (__s32).  This
is a bit strange, since (with one exception--fl_pg_preferred):
    - there is no need for negative values; and
    - all users of these macros are assigning their result to
      64-bit variables.
So just make these macros return a 64-bit unsigned type.

The exception is the preferred placement group, which remains a
signed 32-bit value.  A placement group id encodes the preferred
primary OSD in a 16-bit value, and there's no sense at this point
getting too far away from that.

And finally, the fl_cas_hash and fl_object_stripe_unit fields are
essentially unused, but are both 32 bits and signed.  Keep their
size, but make them unsigned, since neither a hash nor a stripe
unit has any business having a negative value.

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