Automounting nfs share fails with err=2, RPC: Timed out
Your Arch Linux distribution probably contains an NFSv4 server. The NFSv4 server requires a base directory (e.g. /srv/nfs4).

Any additional share has to be a subdirectory (e.g. /srv/nfs4/home_tmp). So move the server-side /home_tmp to the base directory.

The file /etc/exports looks like this then:

/srv/nfsv4 192.168.0.0/26(rw,fsid=0,insecure,no_subtree_check,async)
/srv/nfsv4/home_tmp 192.168.0.0/26(rw,nohide,insecure,no_subtree_check,async)


(only tested with async but should also work with sync)

Reload the file with exportfs -arv on the server.

Your client-side file /etc/auto_nfs has to be changed also:

/home_tmp -fstype=nfs,vers=3,rw,resvport,tcp,soft,intr,rsize=8192,wsize=8192,noatime,timeo=900,retrans=3 192.168.0.2:/srv/nfsv4/home_tmp


Finally enter sudo automount -vc in the client's shell to "refresh" the mounted share.



To set up networked home directories for OS X users some directory integration is required or at least recommended (e.g. LDAP or AD). For Linux servers I've found a pretty old how-to: 
Automount OS X Home Directories Using OpenLDAP and Linux. I don't know if the approach still works well with newer macOS versions. An updated how-to would require a new question here or at Server Fault.