if_epair: build fix

66acf7685b failed to build on riscv (and mips). This is because the
atomic_testandset_int() (and friends) functions do not exist there.
Happily those platforms do have the long variant, so switch to that.

PR: 262571
MFC after: 3 days
Approved by: re (gjb)

(cherry picked from commit 0bf7acd6b7047537a38e2de391a461e4e8956630)