LU-3557 mdt: fix open existing file with O_CREAT for ro mount

LU-3557 mdt: fix open existing file with O_CREAT for ro mount

We should only return -EROFS for read only mount if we really
need to create the file in question.

Defer the check to mdt_reint_open() instead of mdt_intent_opc().

Signed-off-by: Li Dongyang <dongyang.li@anu.edu.au>
Change-Id: I68f34ef7abbae934cfc3564a67ff70e5ab8a12b1
Reviewed-on: http://review.whamcloud.com/6893
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Li Xi <pkuelelixi@gmail.com>
Reviewed-by: Emoly Liu <emoly.liu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>