ovl: fix NULL pointer defer when encoding non-decodable lower fid

A wrong return value from ovl_check_encode_origin() would cause
ovl_dentry_to_fid() to try to encode fid from NULL upper dentry.

Reported-by:  <syzbot+2208f82282740c1c8915@syzkaller.appspotmail.com>
Fixes: 16aac5ad ("ovl: support encoding non-decodable file handles")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>