mm, slub: restructure new page checks in ___slab_alloc()

mm, slub: restructure new page checks in ___slab_alloc()

When we allocate slab object from a newly acquired page (from node's
partial list or page allocator), we usually also retain the page as a new
percpu slab.  There are two exceptions - when pfmemalloc status of the
page doesn't match our gfp flags, or when the cache has debugging enabled.

The current code for these decisions is not easy to follow, so restructure
it and add comments.  The new structure will also help with the following
changes.  No functional change.

Link: https://lkml.kernel.org/r/20210805152000.12817-11-vbabka@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Cc: Christoph Lameter <cl@linux.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Jann Horn <jannh@google.com>
Cc: Jesper Dangaard Brouer <brouer@redhat.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>