LU-540 misc patch for user identity upcall/downcall

LU-540 misc patch for user identity upcall/downcall

1) l_getidentity forgets to allocate supplementary group info space, but access
   such non-allocated memoey without checking.
2) In structure "identity_downcall_data", "idd_gid" is same as "idd_groups[0]",
   remove such redundant information.
3) More sanity check for lprocfs_wr_identity_info().
4) Drop unnecessary memory allocation to simplify mdt_identity_do_upcall().
5) Add padding field in "identity_downcall_data" to resolve align issues.
6) Use new magic to distinguish old version/format downcall.

Change-Id: Ib0dfb37c39a7cb335e29557b68b885afa726789e
Signed-off-by: nasf <yong.fan@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/1160
Tested-by: Hudson
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>