page_pool: help compiler remove code in case CONFIG_NUMA=n

When kernel is compiled without NUMA support, then page_pool NUMA
config setting (pool->p.nid) doesn't make any practical sense. The
compiler cannot see that it can remove the code paths.

This patch avoids reading pool->p.nid setting in case of !CONFIG_NUMA,
in allocation and numa check code, which helps compiler to see the
optimisation potential. It leaves update code intact to keep API the
same.

 $ ./scripts/bloat-o-meter net/core/page_pool.o-numa-enabled \
                           net/core/page_pool.o-numa-disabled
 add/remove: 0/0 grow/shrink: 0/3 up/down: 0/-113 (-113)
 Function                                     old     new   delta
 page_pool_create                             401     398      -3
 __page_pool_alloc_pages_slow                 439     426     -13
 page_pool_refill_alloc_cache                 425     328     -97
 Total: Before=3611, After=3498, chg -3.13%

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>