net: fix comment typo for __skb_alloc_pages()

The name of the function in the comment is __skb_alloc_page() while we
are actually commenting __skb_alloc_pages(). Fix this typo and make it
a valid kernel doc comment.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>