sysfstools: file_write: properly catch errors

since `print` is doing buffered IO, we don't always get an error there,
even if the underlying write does not work.

To properly catch that, do an unbuffered `syswrite` which circumvents
all buffers and writes directly to the file handle.

We aren't actually interested in the specific error here, but only if
the write was successful or not.

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>
Tested-by: Christoph Heiss <c.heiss@proxmox.com>

src/PVE/SysFSTools.pm

@@ -198,7 +198,7 @@ sub file_write {
     my $fh = IO::File->new($filename, "w");
     return undef if !$fh;
 
-    my $res = print $fh $buf;
+    my $res = defined(syswrite($fh, $buf)) ? 1 : 0;
 
     $fh->close();