LU-2463 security: osd_mknod shouldn't alter mode after creation

ldiskfs osd_mknod() calls init_special_inode() after creation, which
will update inode->i_mode, don't use the mode from client because
mode is decided at creation time.

Test-Parameters: envdefinitions=SLOW=yes testlist=posix
Signed-off-by: Lai Siyao <laisiyao@whamcloud.com>
Change-Id: Id11c00bf57903e210fc96d33437dd3ece6f2e171
Reviewed-on: http://review.whamcloud.com/4829
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>