gve: Fix skb truesize underestimation

For a skb frag with a newly allocated copy page, the true size is
incorrectly set to packet buffer size. It should be set to PAGE_SIZE
instead.

Fixes: 82fd151d ("gve: Reduce alloc and copy costs in the GQ rx path")
Signed-off-by: Praveen Kaligineedi <pkaligineedi@google.com>
Link: https://lore.kernel.org/r/20240124161025.1819836-1-pkaligineedi@google.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>