ip_ecn: don't touch the DSCP bits and flow_id

[p.mousavizadeh_protonmail.com]

Summary

don't touch the DSCP bits
in ip_ecn_ingress, ip6_ecn_ingress and ip6_ecn_egress
except for NO_CARE for backward compatibility.
raised in D53516

Test Plan

ipsec, gif and stf should be tested.

[p.mousavizadeh_protonmail.com]

sys/netinet/ip_ecn.c

133–135

@rscheff
I think the NOCARE is redundant. I copied the inner bits to the outer for ECN_NOCARE mode to preserve ipv4 previous behaviour, but I couldn't preserve backward compatibility for ipv6 without extra complexity due to ECN bit masking in ipv6 wrapper.
I handled IPv4 this way because I thought those who use this mode probably don't know how to handle DSCP and ECN fields. However, I removed the copying part from ECN_FORBIDDEN and IMHO consumers must handle their DSCP bits differently.

IMHO it's better to remove ECN_NOCARE mode, because net developers must decide if they want to support ECN for their module or not.