pstore: Run after modules are loaded

pstore: Run after modules are loaded

The systemd-pstore service takes pstore files on boot and transfers them
to disk. It only does it once on boot and only if it finds any. The typical
location of the pstore on modern systems is the UEFI variable store.

Most distributions ship with CONFIG_EFI_VARS_PSTORE=m. That means, the
UEFI variable store is only available on boot after the respective module
is loaded.

In most situations, the pstore service gets loaded before the UEFI pstore,
so we don't get to transfer logs. Instead, they accumulate, filling up the
pstore over time, potentially breaking the UEFI variable store.

Let's add a service dependency on any kernel module that can provide a
pstore to ensure we only scan for pstate after we can actually see pstate.

I have seen live occurences of systems breaking because we did not erase
the pstates and ran out of UEFI nvram space.

Fixes https://github.com/systemd/systemd/issues/18540