net/packet: remove unused PGV_FROM_VMALLOC definition.

This patch removes the definition of PGV_FROM_VMALLOC from af_packet.c.
The PGV_FROM_VMALLOC definition was already removed by
commit 441c793a ("net: cleanup unused macros in net directory"),
and its usage was removed even before by commit c56b4d90
("af_packet: remove pgv.flags"); but it was added back by mistake later on,
in commit f6fb8f10 ("af-packet: TPACKET_V3 flexible buffer implementation").

Signed-off-by: Rami Rosen <rami.rosen@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>