Revert "if_epair: rework"

Revert the recent performance rework of if_epair. It relies on functions like
atomic_testandclear_long() which are not available on all platforms in
stable/12.

This reverts commits b1a3f8dccb6203036b7ee81201fd5b5a8de36f0d,
fb3644ab2afe777fdd2539bc996a390443f052f1,
ca7af63e88f8cc96865d45e020a57b3062631388,
092da35a0d80af7a3e5c5c22cbeddb6cffbd9524,
and 7c2b681b33fc78ed06c7e9e65eeebb2ab5420586.

This is a direct commit to stable/12.