ceph: make ceph file layout helpers take a pointer

ceph: make ceph file layout helpers take a pointer

Make the helpers for accessing the fields of an on-disk ceph file
layout structure take the address of a layout structure rather than
the structure itself.  It's more useful (and to my eye a bit more
normal).

Use the result in ceph_file_layout_stripe_width() and
ceph_file_layout_period(), since it's now possible/natural.

Rename ceph_file_layout_su() to be ceph_file_layout_stripe_unit().

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