arm64: ptrace: fix compat reg getter/setter return values

copy_{to,from}_user return the number of bytes remaining on failure, not
an error code.

This patch returns -EFAULT when the copy operation didn't complete,
rather than expose the number of bytes not copied directly to userspace.
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/kernel/ptrace.c

@@ -663,8 +663,10 @@ static int compat_gpr_get(struct task_struct *target,
 			kbuf += sizeof(reg);
 		} else {
 			ret = copy_to_user(ubuf, &reg, sizeof(reg));
-			if (ret)
+			if (ret) {
+				ret = -EFAULT;
 				break;
+			}
 
 			ubuf += sizeof(reg);
 		}
@@ -702,8 +704,10 @@ static int compat_gpr_set(struct task_struct *target,
 			kbuf += sizeof(reg);
 		} else {
 			ret = copy_from_user(&reg, ubuf, sizeof(reg));
-			if (ret)
-				return ret;
+			if (ret) {
+				ret = -EFAULT;
+				break;
+			}
 
 			ubuf += sizeof(reg);
 		}