page_pool: rename __page_pool_alloc_pages_slow() to __page_pool_alloc_netmems_slow()

page_pool: rename __page_pool_alloc_pages_slow() to __page_pool_alloc_netmems_slow()

Now that __page_pool_alloc_pages_slow() is for allocating netmem, not
struct page, rename it to __page_pool_alloc_netmems_slow() to reflect
what it does.

Signed-off-by: Byungchul Park <byungchul@sk.com>
Reviewed-by: Mina Almasry <almasrymina@google.com>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://patch.msgid.link/20250702053256.4594-4-byungchul@sk.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>