selftests: drv-net-hw: inject pp_alloc_fail errors in the right place

selftests: drv-net-hw: inject pp_alloc_fail errors in the right place

The tool pp_alloc_fail.py tested error recovery by injecting errors
into the function page_pool_alloc_pages(). The page pool allocation
function page_pool_dev_alloc() does not end up calling
page_pool_alloc_pages(). page_pool_alloc_netmems() seems to be the
function that is called by all of the page pool alloc functions in
the API, so move error injection to that function instead.

Signed-off-by: John Daley <johndale@cisco.com>
Link: https://patch.msgid.link/20250115181312.3544-2-johndale@cisco.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>