af_packet: use vmalloc_to_page() instead for the addresss returned by vmalloc()

The following commit causes the pgv->buffer may point to the memory
returned by vmalloc(). And we can't use virt_to_page() for the vmalloc
address.

This patch introduces a new inline function pgv_to_page(), which calls
vmalloc_to_page() for the vmalloc address, and virt_to_page() for the
__get_free_pages address.

We used to increase page pointer to get the next page at the next page
address, after Neil's patch, it is wrong, as the physical address may
be not continuous. This patch also fixes this issue.

    commit 0e3125c7
    Author: Neil Horman <nhorman@tuxdriver.com>
    Date:   Tue Nov 16 10:26:47 2010 -0800

    packet: Enhance AF_PACKET implementation to not require high order contiguous memory allocation (v4)

Signed-off-by: Changli Gao <xiaosuo@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>