page_pool: fix compile warning when CONFIG_PAGE_POOL is disabled

Kbuild test robot reported compile warning:
 warning: no return statement in function returning non-void
in function page_pool_request_shutdown, when CONFIG_PAGE_POOL is disabled.

The fix makes the code a little more verbose, with a descriptive variable.

Fixes: 99c07c43 ("xdp: tracking page_pool resources and safe removal")
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>