ceph: make use of ceph file layout helpers

ceph: make use of ceph file layout helpers

There are helpers defined in "include/linux/ceph/osdmap.h" for
accessing the fields of an on-disk ceph file layout structure.
Use them--consistently--throughout the code.

Also define CEPH_FILE_LAYOUT_PG_PREFERRED_NONE, to represent
symbolically the explicit "no preferred PG" value.

Make a few casts explicit too (to make it more obvious it's
occuring).  This produces some long lines, but they go away in an
upcoming patch.

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