mm: make folio page count functions return unsigned

As raised by Andrew [1], a folio/compound page never spans a negative
number of pages.  Consequently, let's use "unsigned long" instead of
"long" consistently for folio_nr_pages(), folio_large_nr_pages() and
compound_nr().

Using "unsigned long" as return value is fine, because even
"(long)-folio_nr_pages()" will keep on working as expected.  Using
"unsigned int" instead would actually break these use cases.

This patch takes the first step changing these to return unsigned long
(and making drm_gem_get_pages() use the new types instead of replacing
min()).

In the future, we might want to make more callers of these functions to
consistently use "unsigned long".


Link: https://lore.kernel.org/linux-mm/20250503182858.5a02729fcffd6d4723afcfc2@linux-foundation.org/
Link: https://lkml.kernel.org/r/20250826153721.GA23292@cathedrallabs.org
Link: https://lore.kernel.org/linux-mm/20250503182858.5a02729fcffd6d4723afcfc2@linux-foundation.org/

 [1]
Signed-off-by: Aristeu Rozanski <aris@ruivo.org>
Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Suggested-by: David Hildenbrand <david@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@gmail.com>
Cc: Simona Vetter <simona@ffwll.ch>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>