net/ipv6: ioam6: prevent schema length wraparound in trace fill

ioam6_fill_trace_data() stores the schema contribution to the trace
length in a u8. With bit 22 enabled and the largest schema payload,
sclen becomes 1 + 1020 / 4, wraps from 256 to 0, and bypasses the
remaining-space check. __ioam6_fill_trace_data() then positions the
write cursor without reserving the schema area but still copies the
4-byte schema header and the full schema payload, overrunning the trace
buffer.

Keep sclen in an unsigned int so the remaining-space check and the write
cursor calculation both see the full schema length.

Fixes: 8c6f6fa67726 ("ipv6: ioam: IOAM Generic Netlink API")
Signed-off-by: Pengpeng Hou <pengpeng@iscas.ac.cn>
Reviewed-by: Justin Iurman <justin.iurman@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>