cifs: validate idmap key payload length

cifs: validate idmap key payload length

The cifs.idmap key type stores its payload length in key->datalen, which
is limited to U16_MAX.  Accepting a larger key payload truncates the
recorded length and can make later users interpret the payload using
inconsistent bounds.

Reject oversized preparsed payloads before allocating or copying them.
This keeps key->datalen consistent with the stored data for both inline
and separately allocated idmap payloads.

Signed-off-by: Li Qiang <liqiang01@kylinos.cn>
Signed-off-by: Steve French <stfrench@microsoft.com>