linux(4): Fix get_robust_list() syscall return value.

The system call returns the head of the robust futex list. The list head is stored
in the location pointed to by the head argument. When copying data between address
spaces use proper head storage size as it depends on an emulated ABI.

PR: 267616
MFC after: 3 days

(cherry picked from commit 9f7bf94ee2a88a702cef9fdb67166404c275afaf)