This aligns the definition of our user copy services with the kernel in
two aspects:
 - use unlikely to mark the fast path
 - memset the remaining bits of the target region if the copy fails;
   this avoids false-positive compiler warnings and reduces the risk to
   evaluate undefined data in case the return value is not checked

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>