fork_worker: use correct handle type for POSIX::write

$resfh can be a pipe from POSIX::pipe() or the upid output
handle, which is an IO::File, so we need to take its
fileno().

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
Fixes: ed52a8435a ("fork_worker: use separate pipe for status messages")
Reviewed-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

src/PVE/RESTEnvironment.pm

@@ -525,7 +525,7 @@ sub fork_worker {
 		    if !open(STDIN, "</dev/null");
 
 		$outfh = PVE::Tools::upid_open($upid);
-		$resfh = $outfh;
+		$resfh = fileno($outfh);
 	    }