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

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

osd_mknod() calls init_special_inode(), but it should use
inode->i_mode instead of the one from client.

Test-Parameters: envdefinitions=SLOW=yes testlist=posix
Signed-off-by: Lai Siyao <laisiyao@whamcloud.com>
Change-Id: Iee0f49dc4beef4aae6b5b6f57679de2643887e25
Reviewed-on: http://review.whamcloud.com/4820
Tested-by: Hudson
Reviewed-by: Jian Yu <jian.yu@intel.com>
Tested-by: Jian Yu <jian.yu@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>