9pfs: local: fix unlink of alien files in mapped-file mode

When trying to remove a file from a directory, both created in non-mapped
mode, the file remains and EBADF is returned to the guest.

This is a regression introduced by commit "df4938a6651b 9pfs: local:
unlinkat: don't follow symlinks" when fixing CVE-2016-9602. It changed the
way we unlink the metadata file from

    ret = remove("$dir/.virtfs_metadata/$name");
    if (ret < 0 && errno != ENOENT) {
         /* Error out */
    }
    /* Ignore absence of metadata */

to

    fd = openat("$dir/.virtfs_metadata")
    unlinkat(fd, "$name")
    if (ret < 0 && errno != ENOENT) {
         /* Error out */
    }
    /* Ignore absence of metadata */

If $dir was created in non-mapped mode, openat() fails with ENOENT and
we pass -1 to unlinkat(), which fails in turn with EBADF.

We just need to check the return of openat() and ignore ENOENT, in order
to restore the behaviour we had with remove().

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
[groug: rewrote the comments as suggested by Eric]
