You need the compatibility layer. Do the following:

linux_compatibility.sh

kldload linprocfs
cat 'linproc /compat/linux/proc linprocfs rw,late 0 0' >> /etc/fstab
mkdir -p /usr/compat/linux/proc
ln -s /usr/compat /compat
mount linproc

See https://forums.freebsd.org/threads/29559/