[PATCH v8] PPTP: PPP over IPv4 (Point-to-Point Tunneling Protocol)

PPP: introduce "pptp" module which implements point-to-point tunneling protocol using pppox framework
NET: introduce the "gre" module for demultiplexing GRE packets on version criteria 
     (required to pptp and ip_gre may coexists)
NET: ip_gre: update to use the "gre" module

Signed-off-by: Dmitry Kozlov <redacted>
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This patch introduces then pptp support to the linux kernel which dramatically speeds up pptp vpn connections and decreases cpu usage in comparison of existing user-space implementation (poptop/pptpclient). There is accel-pptp project (https://sourceforge.net/projects/accel-pptp/) to utilize this module, it contains plugin for pppd to use pptp in client-mode and modified pptpd (poptop) to build high-performance pptp NAS.

There was many changes from initial submitted patch, most important are:
1. using rcu instead of read-write locks
2. using static bitmap instead of dynamically allocated
3. using vmalloc for memory allocation instead of BITS_PER_LONG + __get_free_pages
4. fixed many coding style issues
Thanks to Eric Dumazet.