`networking.firewall.checkReversePath` was being set to "loose" from
Mullvad VPN, which was causing an issue with the kernel used by the
PinePhone with Mobile NixOS.
By changing this option to `false`, we get rid of the "This kernel does
not support rpfilter" error, which seems to be inaccurate due to the
result of `sysctl -a | grep \\.rp_filter` on the phone being consistent
with the result on the laptop.
