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: ed52a8435a6d ("fork_worker: use separate pipe for status messages")
Reviewed-by: Thomas Lamprecht <t.lamprecht@proxmox.com>