mm/slab.c: cleanup is_debug_pagealloc_cache()

Remove the if statement to increase code readability.
Also make the function inline, per David.

Signed-off-by: lvqian <lvqian@nfschina.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>