ceph: implement hidden per-field ceph.*.layout.* vxattrs

Allow individual fields of the layout to be fetched via getxattr.
The ceph.dir.layout.* vxattr with "disappear" if the exists_cb
indicates there no dir layout set.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Sam Lang <sam.lang@inktank.com>