net: simplify sk_page_frag

Now that in-kernel socket users that may recurse during reclaim have benn
converted to sk_use_task_frag = false, we can have sk_page_frag() simply
check that value.

Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>