client: don't include ceph.{file,dir}.layout vxattr in listxattr()

This avoids 'cp -a' modifying layout of new files/directories.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Merge pull request #1525 from ceph/wip-listxattr

client: don't include ceph.{file,dir}.layout vxattr in listxattr()

Reviwed-by: Sage Weil <sage@inktank.com>