x86: streamline copying to/from user memory

x86: streamline copying to/from user memory

Their size parameters being "unsigned", there's neither a point for
them returning "unsigned long", nor for any of their (assembly)
arithmetic to involved 64-bit operations on other than addresses.

Take the opportunity and fold __do_clear_user() into its single user
(using qword stores instead of dword ones), name all asm() operands,
and reduce the amount of (redundant) operands.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>