Instead doing the check for allocation in each loop, move it outside
the while loop and do it every NAPI loop.

This change boosts the xdpsock rxdrop scenario with 15% more
packets-per-second.
Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Link: https://lore.kernel.org/r/20201211085410.59350-1-bjorn.topel@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>