mm/gup: fix out-of-bounds write for nr_pages == 0 in GUP-fast

With nr_pages == 0, GUP-fast still enters its page table walk and can
write past the caller's page array. The old end <= start check caught
empty ranges, but was replaced with an overflow check.

Users such as OrangeFS can pass a zero page count derived from a
userspace-supplied length. So let's return zero for nr_pages == 0 before
starting the walk, just like GUP-slow.

Fix generated by an LLM and sent by Chris Mason in private.

Fixes: c28b1fc703 ("mm/gup: reorganize internal_get_user_pages_fast()")
Cc: stable@vger.kernel.org
Signed-off-by: Chris Mason <mason@kernel.org>
[ david: simplify description ]
Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Acked-by: Muchun Song <muchun.song@linux.dev>
Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Link: https://patch.msgid.link/20261031-fixes_core-v3-21-d6437171ecc3@kernel.org
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>

mm/gup.c

@@ -3215,6 +3215,9 @@ static int gup_fast_fallback(unsigned long start, unsigned long nr_pages,
 				       FOLL_PCI_P2PDMA | FOLL_HONOR_NUMA_FAULT)))
 		return -EINVAL;
 
+	if (unlikely(!nr_pages))
+		return 0;
+
 	if (gup_flags & FOLL_PIN)
 		mm_set_has_pinned_flag(current->mm);