net/tls: fix sign extension issue when left shifting u16 value

Left shifting the u16 value promotes it to a int and then it
gets sign extended to a u64.  If len << 16 is greater than 0x7fffffff
then the upper bits get set to 1 because of the implicit sign extension.
Fix this by casting len to u64 before shifting it.

Addresses-Coverity: ("integer handling issues")
Fixes: ed9b7646 ("net/tls: Add asynchronous resync")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>