the netlink macro NETLINK_GENERIC is conflicted with the old name defined in if_mib.h

[steven_chen3_dell.com]

Summary

the old name of NETLINK_GENERIC is used by interface MIB type query in SNMP, either we change the name of the old feature type, or we change the name of the new feature type. in order to application code cross platform porting, we keep netlink name is the same as linux, so we change the MIB type name macro name to NETLINK_GENERIC_MIB.

[steven_chen3_dell.com]

select this commit or https://reviews.freebsd.org/D41820