LU-4101 mdt: protect internal xattrs

LU-4101 mdt: protect internal xattrs

In mdt_reint_setxattr() require CAP_SYS_ADMIN to modify a trusted
xattr and silently disallow modification those trusted xattrs used by
Lustre internally.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: Ic616dca74a90da0aedb0ec2624618f91ac6fcaf4
Reviewed-on: http://review.whamcloud.com/7943
Reviewed-by: Andrew Perepechko <andrew_perepechko@xyratex.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>