slub/kunit: skip test_kfree_rcu when the slub kunit test is built-in

Guenter Roeck reports that the new slub kunit tests added by commit
4e1c44b3db79 ("kunit, slub: add test_kfree_rcu() and
test_leak_destroy()") cause a lockup on boot on several architectures
when the kunit tests are configured to be built-in and not modules.

The test_kfree_rcu test invokes kfree_rcu() and boot sequence inspection
showed the runner for built-in kunit tests kunit_run_all_tests() is
called before setting system_state to SYSTEM_RUNNING and calling
rcu_end_inkernel_boot(), so this seems like a likely cause. So while I
was unable to reproduce the problem myself, skipping the test when the
slub_kunit module is built-in should avoid the issue.

An alternative fix that was moving the call to kunit_run_all_tests() a
bit later in the boot was tried, but has broken tests with functions
marked as __init due to free_initmem() already being done.

Fixes: 4e1c44b3db79 ("kunit, slub: add test_kfree_rcu() and test_leak_destroy()")
Reported-by: Guenter Roeck <[email protected]>
Closes: https://lore.kernel.org/all/[email protected]/
Cc: Paul E. McKenney <[email protected]>
Cc: Boqun Feng <[email protected]>
Cc: Uladzislau Rezki <[email protected]>
Cc: [email protected]
Cc: Brendan Higgins <[email protected]>
Cc: David Gow <[email protected]>
Cc: Rae Moar <[email protected]>
Cc: [email protected]
Cc: [email protected]
Tested-by: Guenter Roeck <[email protected]>
Signed-off-by: Vlastimil Babka <[email protected]>

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