l2tp: fix incorrect parameter validation in the pppol2tp_getsockopt() function

[ Upstream commit 955e9876ba4ee26eeaab1b13517f5b2c88e73d55 ]

The 'len' variable can't be negative when assigned the result of
'min_t' because all 'min_t' parameters are cast to unsigned int,
and then the minimum one is chosen.

To fix the logic, check 'len' as read from 'optlen',
where the types of relevant variables are (signed) int.

Fixes: 3557baab ("[L2TP]: PPP over L2TP driver core")
Reviewed-by: Tom Parkin <tparkin@katalix.com>
Signed-off-by: Gavrilov Ilia <Ilia.Gavrilov@infotecs.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>