LU-2780 mdd: use a real inode for .lustre/fid

LU-2780 mdd: use a real inode for .lustre/fid

Use a real inode .lustre/fid thereby allowing traditional DACs to
govern open by fid.  Set the default mode of .lustre/fid to
0100/d--x------.  Remove the prohibition on setting attributes of
.lustre and .lustre/fid in mdt_reint_setattr().  Don't set IMMUTE_OBJ
in .lustre/fid's mod_flags and remove the check for IMMUTE_OBJ in
mdd_create_data().  Add a check to sanity test_154 to ensure that
setattr works on .lustre/fid.

Intel-bug-id: INTL-5
Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: I372e9d6bb230e5b078dbf028fdc4348dfa192f93
Reviewed-on: http://review.whamcloud.com/5298
Tested-by: Hudson
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>