smb: client: ensure open_cached_dir_by_dentry() only returns valid cfid

smb: client: ensure open_cached_dir_by_dentry() only returns valid cfid

open_cached_dir_by_dentry() was exposing an invalid cached directory to
callers. The validity check outside the function was exclusively based
on cfid->time.

Add validity check before returning success and introduce
is_valid_cached_dir() helper for consistent checks across the code.

Signed-off-by: Henrique Carvalho <henrique.carvalho@suse.com>
Reviwed-by: Enzo Matsumiya <ematsumiya@suse.de>
Signed-off-by: Steve French <stfrench@microsoft.com>