so_txtime.sh in net from ubuntu_kernel_selftests failed with "exceeds variance" on RISCV64

[Po-Hsu Lin]

Issue found on RISCV64 instance with:
  * Impish RISCV 5.13.0-1007.7
  * Hirsute RISCV 5.11.0-1024.25
  * F-5.11 RISCV 5.11.0-1024.25~20.04.1

Test log:
 TAP version 13
 1..1
 # selftests: net: so_txtime.sh
 #
 # SO_TXTIME ipv4 clock monotonic
 # payload:a delay:4306 expected:0 (us)
 # ./so_txtime: exceeds variance (4000 us)
 not ok 1 selftests: net: so_txtime.sh # exit=1

This is not a regression, it has failed in the previous cycle as well. It's just because we have the whole "net" test hinted so this was never reported.

[Po-Hsu Lin (cypressyew)]

Found on J-allwinner-5.19.0-1007.7~22.04.1