bpf: verifier (add docs)

this patch adds all of eBPF verfier documentation and empty bpf_check()

The end goal for the verifier is to statically check safety of the program.

Verifier will catch:
- loops
- out of range jumps
- unreachable instructions
- invalid instructions
- uninitialized register access
- uninitialized stack access
- misaligned stack access
- out of range stack access
- invalid calling convention

More details in Documentation/networking/filter.txt

Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/bpf.h

@@ -83,5 +83,7 @@ struct bpf_prog_aux {
 
 void bpf_prog_put(struct bpf_prog *prog);
 struct bpf_prog *bpf_prog_get(u32 ufd);
+/* verify correctness of eBPF program */
+int bpf_check(struct bpf_prog *fp, union bpf_attr *attr);
 
 #endif /* _LINUX_BPF_H */