ksmbd: fix SMB 3.11 posix extension mount failure

cifs client set 4 to DataLength of create_posix context, which mean
Mode variable of create_posix context is only available. So buffer
validation of ksmbd should check only the size of Mode except for
the size of Reserved variable.

Fixes: 8f77150c ("ksmbd: add buffer validation for SMB2_CREATE_CONTEXT")
Cc: stable@vger.kernel.org # v5.15+
Reported-by: Steve French <smfrench@gmail.com>
Tested-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>