LU-13525 sec: better struct sepol_downcall_data

LU-13525 sec: better struct sepol_downcall_data

struct sepol_downcall_data is badly formed for several reasons:
- it uses a __kernel_time_t field, which can be variably sized,
  depending on the size of __kernel_long_t. Replace it with a
  fixed-size __s64 type;
- it has __u32 sdd_magic that is immediately before a potentially
  64-bit field, whereas the 64-bit fields in a structure should
  always be naturally aligned on 64-bit boundaries to avoid potential
  incompatibility in the structure definition;
- it has __u16 sdd_sepol_len which may be followed by padding.

So create a better struct sepol_downcall_data, while maintaining
compatibility with 2.12 by keeping a struct sepol_downcall_data_old.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I07c573c2eef64fb0c796d8af4acdc3428e0761a8
Reviewed-on: https://review.whamcloud.com/38580
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Olaf Faaland-LLNL <faaland1@llnl.gov>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>