9p: Uninitialized variable in v9fs_writeback_fid()

If v9fs_fid_lookup_with_uid() fails then "fid" is not initialized.

The v9fs_fid_lookup_with_uid() can't return NULL.  If it returns an
error pointer then we can still pass that to clone_fid() and it will
return the error pointer back again.

Fixes: 6636b6dc ("9p: add refcount to p9_fid struct")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>