9p: v9fs_fid_find: also lookup by inode if not found dentry

It's possible for v9fs_fid_find "find by dentry" branch to not turn up
anything despite having an entry set (because e.g. uid doesn't match),
in which case the calling code will generally make an extra lookup
to the server.

In this case we might have had better luck looking by inode, so fall
back to look up by inode if we have one and the lookup by dentry failed.

Message-Id: <20240523210024.1214386-1-asmadeus@codewreck.org>
Reviewed-by: Christian Schoenebeck <linux_oss@crudebyte.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>