mm/show_mem: use str_yes_no() helper in show_free_areas()

Remove hard-coded strings by using the str_yes_no() helper function.

Link: https://lkml.kernel.org/r/20241026103552.6790-2-thorsten.blum@linux.dev


Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>