Description of problem:

When having users with home dire on NFS and enabling lingering for such users, systemd fails to start the user@ service instance for the lingering user because the home dir is not present yet. This ends up not having the user services started.

IMHO user@.service should have the following property added, so that it gets delayed until home dir is available:
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------
RequiresMountsFor=%h
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------

Unfortunately due to BZ #2056527 "%h" cannot be used for now.

Version-Release number of selected component (if applicable):

systemd-239

How reproducible:

Always

Steps to Reproduce:
1. Create a user with home dir on NFS

2. Enable lingering for that user
3. Create a user service (/home/rmetrich/.config/systemd/user/test.service) and enable it

-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------
[Service]
ExecStart=/bin/sleep 3600

[Install]
WantedBy=default.target
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------

4. Reboot

Actual results:

"systemd --user" loads but "test.service" unit is not known / didn't execute because "systemd --user" instance has a "null" configuration:
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------
$ systemctl --user status test
Unit test.service could not be found.
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------

Expected results:

"test.service" executes.

Additional info:

With the added dependency on home dir (hardcoded because "%h" fails), this works fine:
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1. systemctl cat user@$(id -u rmetrich)
   [...]
2. /etc/systemd/system/user@.service.d/home.conf
   [Unit]
   RequiresMountsFor=/home/rmetrich

$ systemctl --user status test
● test.service
Loaded: loaded (/home/rmetrich/.config/systemd/user/test.service; enabled; vendor preset: enabled)
Active: active (running) since Mon 2022-02-21 14:24:54 CET; 24s ago
Main PID: 1265 (sleep)
CGroup: /user.slice/user-101166.slice/user@101166.service/test.service
└─1265 /bin/sleep 3600
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------