mbuf: enable ext_pgs ("unmapped") mbufs by default

Ext_pg mbufs allow carrying multiple pages per mbuf. This
reduces mbuf linked list traversals, especially in socket
buffers, thereby reducing cache misses and CPU use for
applications using sendfile. Note that ext_pages use
unmapped pages, eliminating KVA mapping costs on 32-bit
platforms.

Ext_pg mbufs are also required for ktls (KERN_TLS), and having
them disabled by default is a stumbling block for those
wishing to enable ktls.

Reviewed-by: jhb, glebius
Sponsored by: Netfix