samples/damon/prcl: fix boot time enable crash

If 'enable' parameter of the 'prcl' DAMON sample module is set at boot
time via the kernel command line, memory allocation is tried before the
slab is initialized.  As a result kernel NULL pointer dereference BUG can
happen.  Fix it by checking the initialization status.

Link: https://lkml.kernel.org/r/20250706193207.39810-3-sj@kernel.org


Fixes: 2aca2546 ("samples/damon: introduce a skeleton of a smaple DAMON module for proactive reclamation")
Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>