treewide: compare syscall.Errno directly instead of interface type

Change-Id: I03eba72177113cf40afc38981c5311a8013855f9
Reviewed-on: https://review.monogon.dev/c/monogon/+/3042
Reviewed-by: Leopold Schabel <leo@monogon.tech>
Tested-by: Jenkins CI
diff --git a/metropolis/pkg/fsquota/quotactl/quotactl.go b/metropolis/pkg/fsquota/quotactl/quotactl.go
index 9f44ab3..c7939ed 100644
--- a/metropolis/pkg/fsquota/quotactl/quotactl.go
+++ b/metropolis/pkg/fsquota/quotactl/quotactl.go
@@ -107,9 +107,9 @@
 	if err != nil {
 		return err
 	}
-	_, _, err = unix.Syscall6(unix.SYS_QUOTACTL_FD, fd.Fd(), uintptr(Q_QUOTAON|uint(qtype)), uintptr(quotaFormat), uintptr(unsafe.Pointer(pathArg)), 0, 0)
-	if err != unix.Errno(0) {
-		return err
+	_, _, errNo := unix.Syscall6(unix.SYS_QUOTACTL_FD, fd.Fd(), uintptr(Q_QUOTAON|uint(qtype)), uintptr(quotaFormat), uintptr(unsafe.Pointer(pathArg)), 0, 0)
+	if errNo != unix.Errno(0) {
+		return errNo
 	}
 	return nil
 }