Cygwin: fhandler_virtual::exists: always set fileid

Cygwin: fhandler_virtual::exists: always set fileid

Commit a0a25849f9dd ("Cygwin: fhandler_virtual: move fileid to path_conv
member") broke `ls -l /proc/<PID>'.  Turns out, the commit forgot to set
the fileid in case of a virtual root dir (i. e., returning virt_rootdir)
in fhandlers utilizing fileid.  This crashed opendir() due to a random
fileid.

Make sure to set fileid in any case.

Fixes: a0a25849f9dd ("Cygwin: fhandler_virtual: move fileid to path_conv member")
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>