xfrm: use the right specifier for uint64_t

This fixes compile warnings like this:

xfrm/sp.c: In function 'xfrm_sp_dump_line':
xfrm/sp.c:346:3: warning: format '%llu' expects argument of type 'long long unsigned int', but argument 3 has type 'uint64_t' [-Wformat=]
   sprintf (dir, "%llu", sp->lft->soft_byte_limit);

Cc: Thomas Haller <thaller@redhat.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Thomas Haller <thaller@redhat.com>