x86, smap: Do not abuse the [f][x]rstor_checking() functions for user space

With SMAP, the [f][x]rstor_checking() functions are no longer usable
for user-space pointers by applying a simple __force cast.  Instead,
create new [f][x]rstor_user() functions which do the proper SMAP
magic.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Cc: Suresh Siddha <suresh.b.siddha@intel.com>
Link: http://lkml.kernel.org/r/1343171129-2747-3-git-send-email-suresh.b.siddha@intel.com