myri10ge: optimize 4k-boundary check when stocking rx pages

Small optimization to the code which checks to see if we'd cross
a 4K boundary when stocking RX ring.

Signed-off-by: Brice Goglin <brice@myri.com>
Signed-off-by: Andrew Gallatin <gallatin@myri.com>
Signed-off-by: Guillaume Morin <guillaume@morinfr.org>
Signed-off-by: David S. Miller <davem@davemloft.net>