ksmbd: Fix buffer length check in fsctl_validate_negotiate_info()

The validate_negotiate_info_req struct definition includes an extra
field to access the data coming after the header. This causes the check
in fsctl_validate_negotiate_info() to count the first element of the
array twice. This in turn makes some valid requests fail, depending on
whether they include padding or not.

Fixes: f7db8fd0

 ("ksmbd: add validation in smb2_ioctl")
Cc: stable@vger.kernel.org # v5.15
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Acked-by: Hyunchul Lee <hyc.lee@gmail.com>
Signed-off-by: Marios Makassikis <mmakassikis@freebox.fr>
Signed-off-by: Steve French <stfrench@microsoft.com>