mm/cma: use str_plural() in cma_declare_contiguous_multi()

mm/cma: use str_plural() in cma_declare_contiguous_multi()

Use the string choice helper function str_plural() to simplify the code
and to fix the following Coccinelle/coccicheck warning reported by
string_choices.cocci:

  opportunity for str_plural(nr)

Link: https://lkml.kernel.org/r/20250630132318.41339-2-thorsten.blum@linux.dev
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Tested-by: Dev Jain <dev.jain@arm.com>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>