x86/usercopy: Replace open coded stac/clac with __uaccess_{begin, end}

x86/usercopy: Replace open coded stac/clac with __uaccess_{begin, end}

In preparation for converting some __uaccess_begin() instances to
__uacess_begin_nospec(), make sure all 'from user' uaccess paths are
using the _begin(), _end() helpers rather than open-coded stac() and
clac().

No functional changes.

Suggested-by: Ingo Molnar <[email protected]>
Signed-off-by: Dan Williams <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Cc: [email protected]
Cc: Tom Lendacky <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: Al Viro <[email protected]>
Cc: [email protected]
Cc: [email protected]
Link: https://lkml.kernel.org/r/151727416438.33451.17309465232057176966.stgit@dwillia2-desk3.amr.corp.intel.com